When I have sex and am on top I feel a pressure and it hurts too much to keep going. The first time this happened I later realized I was bleeding. What's causing this?

I'm not a doctor, so my suggestion comes from personal experience. The on top position tends to make the guy go deeper in you, around where your cervix is. If you irritate your cervix too much it can bleed. Also when the guy thrusts into you and pushes on the back of your vaginal canal and cervix there's a feeling of pressure. That could be a possibility.