Why did I bleed during sex?

My boyfriend and I have had sex with each other about 5 times, using a condom each time. Both of us were virgins before and did not have any STI's. Everything was going fine and after about 10 minutes he saw blood, we were confused for a minute and realized it was coming from me. I knew I was supposed to get my period within the next few days so we just thought it was that. I put a pad on as usual and we just watched TV, but I wasn't sure why I hadn't got cramps because I always get cramps. About an hour later I noticed my flow had completely stopped, it's been about 6 hours now and still nothing. Please help! Also if it matters we are both 15.