This happened to me. Go to the OBGYN; it could just be a urinary tract infection, or it could be nothing at all. When I went the first time, it was nothing at all. We did everything. Other things it could be:
Cysts on the walls of your vagina
Just a random infection
Tears, maybe not enough lubricant
Vaginitis
Something could be wrong with your ovaries
you might have ripped a little something during sex and thats why it bleeds when you have sex