There are only a limited amount of notes and chords. Nothing you can think of has never been done. Perhaps a song you wrote will be written by somebody else tomorrow.
Some people steal songs. It's unfortunate, but some people are dishonest. Others will steal a song without realizing it. Take the case on George Harrison. On his first solo album was a song that's become a classic: “My Sweet Lord”. However, that song was plagiarized from a sixties song called “He's so fine”. In court, Harrison admitted to stealing it, but without realizing he did it. I won't go in to the details of the case as it's still not wholly resolved and it's very long, but you can read the details on The "My Sweet Lord"/"He's So Fine" Plagarism Suit page. So if an ex-Beatle can do it, anyone can.
For anyone who writes songs protecting them is absolutely necessary. You should find out how to avoid stealing from others and keep others stealing from you.