You really need to see a doctor - pain/bleeding from sex is not normal and you need to get to the bottom of it to rule out anything serious, like an infection or STD, etc.
I have had this - in my case it was because I have a uterus that tilts toward my back (prolapsed uterus - not uncommon) and my boyfriend's penis hit my cervix so it would cause discomfort and cramping and blood. I was so scared and thought I was dying. I am fine. Hope this helps.