Songwriting competitions usually require entries to have no existing legal restictions attached; and to be available for first publication. They want to be sure that, if they award a prize, they won't suddenly find themselves sued by the real copyright owner whose work has been plagiarised (it happens); or by a publishing company which already has rights in the material. If all you have done is put your lyric on a website, then you're probably ok: you'd have to ask the competition organisers in order to be sure, though.

Why on earth didn't you do that in the first place?