What Causes Bleeding And Abdominal Cramping After Having Sex?

My boyfriend and i are trying to have a baby. We just started trying this month. October 8 we started using no protection. I've been having pregnancy symptoms since, and i never have pms like this. However, I had sex with my boyfriend Thursday night 10/20/11 and afterwards i had light pink/beige color from peeing when I wiped. Later it got bright red like a normal period but only very different. I woke up the next morning with very painful cramping/abdomen pain. It wasn't bad enough to make me cry but it hurt and was not like any cramps ive had before. I'm still bleeding, not as heavy as a normal period though and there's really only a whole lot on the pad when i wipe. I also have slight backaching sometimes. The cramping has stopped as of right now. Can this be implantation?