I've also worried about that. I've written lines that I've really liked and then thought, "Oh my God. Is that really mine? Or was it lurking in my subconcious from one of the thousand novels I've read?
But I've since relaxed somewhat. I think Exir is correct that a single line can't be copyrighted. If you imagine the gazillion novels that have been published throughout time, odds suggest that there must be numerous duplicate sentences and even similar paragraphs out there.
I might be totally wrong, but I think the authors who have been found guilty of plagarism had novels that had numerous instances of very close paragraphs from the same source. Too many to seem like mere coincidence.