Hi,
You've got copyright on the song, assuming you wrote it, its automatic as soon as its created in a tangible form.
The only question is whether you can prove it if the need arises. Did you send it via email that you have a record of, is the song online anywhere that you could point to, have other people heard it that could swear to it, do you have early drafts of it???
None of this is proof but it would certainly help.
Did you send it unsolicited or was the publisher expecting it? If the former, they're unlikely to listen to it anyway.
If you're worried, I would just gather whatever evidence you've got, but to be honest, I wouldn't worry.
Yodasdad