How long do you bleed? Is it only during intercourse?
That has happened to me before. Most likely your guy was alittle rougher than normal during intercourse or else he just hit a spot that sort of cut you inside. It's usually a painless cut inside of you. In my case it bleeds just for a few minutes or hours and then it stops. If this is the case, there shouldn't be anything to worry about, but if it continues or is worse than my experience, I would get it checked out.