Help me.. Very Confused!!!?

Hey well last Saturday I took a Pregnancy Test and the - line was very very faint..There was not horizontal line at all.. But I have read that it means positive even if there is a very faint -.. If there was a + sign then I definitely couldn't see it.. I'm 18 days late. I have dull back pain, and I have spotted with a couple of clots. I plan to take another test. Are they a such thing as a faint neg even if it's vertical.. and I still be a pregnant or a positive? Please Help Me!! Could I still be pregnant with this? I still haven't started my period!!