black discharge?

So a few days ago I started getting cramps like I was getting my period which didn't make any sense since it just ended about 1 1/2 to 2 weeks ago, but I have endometriosis and am on the pill so I figured it's probably just a weird month because this used to happen regularly so I treated it like any other period, but the discharge has been only like black/ dark brown rather than red and I don't know if it's just old blood or something more serious?? Please help. I'm 16 and a virgin