Maybe there was fertilization, but no implantation and now your body is getting rid of it. Does that make sense? A year ago I was a day late for my period and I am always on time so I took a test. The test had a very very faint positive. However, that very night I started bleeding very light. So the next day I went to the dr and they did a blood test and found the hormone that meant I could be pregnant, but since I was still spotting they wanted to take it again the next day. They took blood I think 3 or 4 times and the levels kept going down and I never did have a "normal" period, just spotting and stuff.
So maybe your body was trying to get pregnant, but for whatever reason it didn't.
I would contact your dr if you're still concerned or wait until next month and see what happens.