Stratocaster action

Firstly, it's unlikely that what you have dine to the bridge has much to do with this issue.
Secondly, it could be a slight dip in the fingerboard from the 10th fret... But it would be something VERY visible if it were enough to really effect your playing.
The most likely scenario, given that you have fiddled with the truss rod, is that the neck pitch is wrong. The neck pitch is the angle that the neck takes when it is joined to the body. If it is under pitched, it tends to fret out on the higher frets, over pitched, the action is high and difficult to play. It's definitely more noticeable up the fingerboard more.
The easiest way to fix a neck pitch issue is to loosen the strings right off, undo the neck bolts and insert a thin shim ( I like to use business cards) into the neck joint at the front or the back to adjust the angle slightly. With the issue you are stating, you will need less angle... So you should shim the front of the neck joint (headstock side...not bridge side).
Remember... A very small movement here can drastically change things at the other end.

If this is a bit confusing, let me know and I'll try and post. Pic or two for you.

Thanks. I read about that remedy somewhere else and was a little timid about trying it. I didn't know what to use as a shim. But, the way you described it and using a credit card piece, I feel like I could make that work. Oh, btw in my original post I made a mistake and asked if you thought my work on the bridge would affect this and I meant nut. I replaced the nut. But I think your diagnosis/remedy is the ticket. I'll let you know how it goes. THANKS !

wouldn't headstock side -make the angle bigger and the action worse? I have been trying to experiment with it and haven't got it riight yet, but that's what appears to be happening and that's what my analysis of it seems to come up with when I try to reason it out with my head.

You are totally right!
I just re read what I typed and had it totally topsy turvy..so:
1. I think a credit card is too thick, use a cardboard business card sliver instead...
2. definitely the bridge side, NOT the headstock side.

Its a bit hit and miss.. you might find that one piece is too slight a difference, it might require 2 or 3 thicknesses.

Sheesh.

It could be the nut. If the nut seats lower in the route then the previous one the issue will compound as you go down the neck. If I were going to try a shim anywhere I'd start with the easiest spot where you're not as likely to ruin the neck, under the nut! Counter-intuitively, tightening the bridge down may have also contributed. Changing the break angle over the saddles in lowering the rear flush to the body, this is always better accomplished with a block and compensating the tension on the spring claw. Setting the trem down surprisingly effects the tone more then people realize, anyone who has played a non-trem strat can attest to this. My personal preference is to put a trem bumper (for lack of a known name) in the cavity. It's basically a "door stop" in the trem cavity that can be set to stop the metal block that the strings run through from over-traveling beyond the resting position but still allows for full motion to slacken the strings. Mine consist of an L shaped bracket screwed into the cavity with a threaded hole for a screw to run through (or a nut and washer if you don't have a tap set). The screw is set perpendicular to the block effectively stopping momentum upon release. $.50 worth of home depot goods.

Here is a good rule of thumb in guitar repair and upkeep: The neck IS the guitar, everything attached to it is ancillary. How a guitar feels plays and sounds is defined more so by the neck than all other components. Mods upgrades and repairs should be conducted around and in reference to a healthy neck, good frets, proper nut, proper tension. Therefore, neck work should be left to professionals where everything else can be attempted in your home with a butter knife, a butane torch, scotch er.... tape, and a cocktail blender. Unless you see obvious issues (the heads come off) start your repairs elsewhere, assume the neck is good. Pickup height, bridge saddles, spring tension, loose screws... have at'em! Anything you do there can be undone for $50 at your local shop once you've run out of items in the kitchen to poke at it with. If you've exhausted all other options and think the truss needs an adjustment or if you've been adjusting you're truss rod every string-change since the 70's no prob. However, if you've never done it, for gadsake... google a reputable source like Dan Irwin or Fender custom shop first. Or buy a book I think they still make those. Otherwise you could find yourself sticking a shim in the wrong spot under some misguided advice... What? You didn't want to turn your strat into a sitar?

Neck shims...sigh... where did they begin and why does every noob want to put them in? An Essay, by Ezra. They started in the 70's, just like every other bad guitar idea, that's where! There were several contributing factors: Firstly, cocaine. It was everywhere, I don't condone it and I don't imbibe but if you didn't have Rumors on vinyl and a toot of the stuff you weren't there. Next, cheap guitars purchased from Montgomery ward's a decade earlier were coming out of attics everywhere and shims got those old babies back in playable windmill ready condition quick. Lastly, aftermarket accessories, mostly those named after an Idaho jeweler named Floyd. The Floyd Rose required a shim to angle the neck correctly to compensate for the bulky mass of METAL!!! oo/ <devil horns that is the OFR.

Shims are NOT a repair. Rarely, a neck requires a shim but not as a repair for an otherwise usable neck, a shim is one component of a system of mods. (unless by "Stratocaster" you meant, "1946 Martian 000") When you change the angle of the neck you are also... Surprise! changing the angle that the bolts holding the neck on bite into the wood. Remember the famous court case of wood v. metal? No? Well, Metal won. I have a beautiful birdseye warmoth start neck I recently picked up that has a split heel because some chowder head decided to put some card stock in the neck pocket. The neck would have been worth $400, but now its going to need a rescue and probably only be worth $100 after that. Don't shim necks, that said, if you do, the neck bolts need to be shortened at the very least. Otherwise or in addition the holes should be filled and re-drilled at the proper angle or a micro tilt device should be installed. The heel may not split today, it may not split when disco comes back but it will weaken the puny amount of wood around the bolt holes and cause pressure that WILL cause the heel to break. Generally this will occur the Tuesday after the guitar becomes sought after as a classic on the must have list of rich hipsters everywhere. But again, unless you're installing a floyd or an ABR on a Jag, you're fixing the wrong thing. And if you're asking for advice on this forum, lets face it, you're probably not installing a floyd anytime soon (I refer those jobs to my friend who is a full time luthier and I have a fully operational work shop.) The best advice I can give is find better advice. Its much better to know why and when to fix something then to know how to do something.

I fixed it so no one would have to sit through my rant twice. I guess by saying, "Failed to submit, try again" it meant "Go ahead, post it twice know-it-all, every one has all day to read your bloated posts twice."

I own a Strat that I bought new about 18 years ago.It's American made- A Standard Strat. Cost about $400 new. Lately the action is a little high from about the 10th fret on up. It's not horrible, but it's not ideal. I have 2 other electrics and 2 acoustics with great action that honestly I never have needed to adjust. I have replaced the nut and tightened the truss rod and done what I can and it has improved but not to my standard where I am real comfortable. A friend of mine told me that strats are notorious for getting a little bow between say the 10th to 16th frets and it might need a fret job to improve things or a new neck altogether. Also, the bridge is a whammy bridge which I don't use and I have tried to stabilize the bridge by tightening in the screws somewhat so it's now basically stationary.

so,
1) do you think what I did with the bridge has caused any of this?
2) what can I do to fix this ?

any input would be helpful. Thanks !

OK.. I can only go by what Im reading... but I don't see where you stepped back, and did some measuring. Most all guitars will bow a neck to some extent.. thats what adjustable truss rods are for. This "A friend of mine told me that strats are notorious for getting a little bow between say the 10th to 16th frets and it might need a fret job to improve things or a new neck altogether. " scares me, and should you also. When working with a neck that has moved around a little you dont tighten the truss rod, you adjust it. First I would stop, step back, and do a little reading up on setting up a guitar. Check the neck, and see how straight it is. Measure the nut to see if all your string heights are correct. Mesure your action, and make adjustments after you have the correct releaf on the neck, and have the nut height right. You might need to adjust the bridge saddles... or maybe not... If nothing else taking it step by step, the guitar will tell you whats off as you go.

True that.

The neck hump thing is perplexing but I think its confused information. Acoustic guitars will ride the heel as they need a neck reset, causing a "bump" at the 12th or 14th... which IS between the 10th and the 16th. And with that reset, you'd need a shim. Voila!