I'm not exactly trying to conceive, but we haven't been preventing all the time, so I figured I would post this here.

I don't know what's going on with my body. I was supposed to start my period on Sunday (3 days ago), but I started spotting that evening. Sometimes I have spotting before my period, but in that case, my period has always come by the next day with full force. So now it's Wednesday, and I'm still spotting with no period. It's not enough to use a tampon, it's just a tiny bit of blood when I wipe. I took a pregnancy test this morning and yesterday, but both were negative. I don't think I'm pregnant, because I estimated ovulation around CD14, and I didn't have unprotected sex until CD17. It's possible I ovulated later, and my friend suggested this could be implantation bleeding, but I never heard of it lasting for 4 days before and I think I would have a positive pregnancy test by now. Any clue what's going on?