Period and PCOS

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I was recently diagnosed with Polycystic Ovaries (PCOS) and 4 mos ago, I was put on The Pill to regulate my periods. (I'm 21 and had my period maybe 5 times since I got my first one at 15!) So I am pretty new at this period thing. 
Since starting the pill, I have been having regular periods. But today, when another started, the blood looks black and I am in pain!  A burning pain. Wondering if anybody else has these issues? If it's black, does that just mean it's a lighter period?