Why Do I Bleed After Having Sex? I Don't Bleed

Why do i bleed after having sex? I don't bleed heavily, it is just a smear of blood seen when i wipe myself. I dont have rough sex nor do i have it that often but the last few times i have noticed blood mixed with semen when i wipe myself straight after or the next day. The blood is coming from me, not my partner. It is very little blood.