Periods

I'm on birth control (ovima28) and have been on birth control for about 2 years now. I had trouble finding the right one but eventually this one seemed to be the best match and I continued with this one ever since. On April 15th I got my period and have had it for over 10 days now. It varies between dark brown (almost looks black) to a light light pink. It'll go away for a couple hours throughout the day or even overnight, I'll think it's gone have sex with my boyfriend and it will come back. What is going on? Help!! TIA!