Help please!!

I've been on birth control for 3 weeks now, I just had sex for the first time since I started birth control. He climaxed inside of me. The morning after I got off the bed and stood up, and there was a lot of blood running down my leg, I was just bleeding like crazy. I already had my period earlier this month but it was very light. Is the bleeding a side effect of the birth control or does that mean that it's something wrong with me.