Odd Spotting: Period, Pregnancy, or Miscarriage?

About a month ago I had unprotected sex with my So. It wasn't planned, it just kind of happened and I completely forgot about it until I started spotting last Sunday for an hour or two. Then it stopped and a few days later happened again. After that I started bleeding more heavily like a period. I stopped bleeding yesterday but still having really bad cramps. I'm not currently on any birth control because I've been on antibiotics for a long time. I have never spotted before. At first I thought maybe it could have been implantation bleeding, my grandmother made a comment about it could be a miscarriage but it may have just been a period. I'm not sure but any advice would be greatly appreciated.