Black stools during the menstrual cycle--from uterus or bowel?

Bowel. Black stools can be caused by dietary changes, supplements or bleeding in the bowel. If this happens every month, you should get it checked out. It could be related to endometriosis but you need to rule out bowel problems like colitis, polyps or even cancer.
? endometriosis. Black stools are usually a sign of blood in the bowel. If the lower bowel bleeds, then red will be seen in or on the stool. Black stools usually mena that the upper digestive tract has blood, as in stomach or upper bowel. Either way black stools can be GI bleed and can be serious and needs to be checked.