I had this when I had cysts on my ovaries. ( well turns out my one ovary and my tube.) The cysts grew from 2 cms to 7.. and before surgery I had that problem with dark Brown and skipped my period .. Talk to your dr!
I can only guess and here's my guess. Somehow old blood from god knows where has found it's exit and maybe it's cuz there's endo in the bladder or bowels. I had this same thing for a couple of periods, then I got, an now have, regular blood...stop....start again with black, foul smelling blood. I call it old blood, cuz if you've ever seen blood dry it looks like that and it makes sense that with the uterine lining all out of whack it isn't shedding right either. The fresh new bright blood might be on the inside of that mass of dry old thick blood, so it came in a big glob?
Do you know that you have endo?
Be sure that your stomach isn't bleeding. Ease up on NSAIDs like advil just in case it is a stomach problem. I get really bad stomach pain on my period every period, but it's better if I eat organic food and raw food.
Love,
Diemyn