Sexual Health - Women :: Bloody Discharge During/after Sex

I am 24, have only had one sexual partner, and have been taking the pill for almost 5 years now. In the last couple of years, I have experienced a thick, brown, bloody discharge during/after sex. It was random at first, but then almost every time. I have changed birth control pills twice due to this condition. Still nothing seems to be changing. It is back to being random rather than every time, but it is still very frustrating to my partner and myself. It's messy and unpredictable. I am wondering why this is happening and if I can stop it? I'm tired of going to the doctor and being told I am fine and all they want to do is switch my birth control method.