When your period is close by, sex can help to bring it down sooner. I am sure most women have experienced something like that. Also, if you boyfriend has a large penis or the sex is too rough, a little bleeding can occur as well. However, my best bet is that you got your period from the sex.
Perhaps after your period you could visit your gyno to ensure that all is well.
Peace and Love,
Yes its my period. I was so afraid because it was my first time ever having sex so close to the date of my period and never was prepared to see blood coming out of me hours after lol. The body is an amazing machine!