VERY TMI BLOODY PAD

Okay so i started my period five days earlier than usual, but it’s okay because I went to the gyno last month for a checkup, they told me I have oligomenorrhea/PCOS and then gave me birth control pills to control my period, since my periods are so irregular this isn’t bothering me but for some reason my blood is black, brown. What the fuck lol why is it nearly black?! It stopped now, the blood is back to red but I was wondering why that was happening. It isn’t the first time too but it’s been awhile since it’s happened and I forgot. I was wondering if this has happened to anyone and if anyone knows why my blood is black/dark brown.