What Causes Black Vaginal Discharge While On Birth Control?

I am having a black discharge that heavy enough to wear a tampon, but enough to wear a pad. Im not suppose to start my period until july 1, so I was a little early when this started yesterday, June 22. There is no pain, but a chance I could be pregnant. I have not been on birth control for six months. I have never had cancer or any other health problems. I am five foot seven, 140 pounds, and 20 years old. Is there a reason I should be concerned?