What Causes Dark Colored Clots During Menstruation?

Hi. I'm 29 years old. I'm 5'3 and weigh 190 pounds. I'm currently on blood pressure medication and take aspirin and a multivitamin daily. I have been on the shot off and on for five years now. Usually when I'm on the birth control shot, I don't have menstral cycles. About two and a half weeks ago, my cycle started and it lasted almost two weeks. For the past week when I wipe after I void, I see a very dark colored substance. Real dark black and sometimes red or brown. What could be causing this?