What Causes Abdominal Pain And Post Coital Bleeding?

i am on birth control and have tried several types including the depo shot. on the depo shot i bled for 9 straight months. i bleed constantly, especially after sex. i get alot of pains that last for days, they think it is swelling considering an anti inflamatory takes the pain away. it is the most intense pain and cramping i have ever felt. i am std free. what is it? please help me