What could cause black stools during the menstrual cycle?

Bleeding ulcer: Anti-inflammatory medications, such as Ibuprofen or naproxen, which are commonly taken for cramps, can cause the lining of the stomach and small intestine to wear down and an ulcer to form. The ulcer can ooze blood, which turns the stool black like tar. Also, pepto bismol (bismuth subsalicylate) or iron supplements are common causes. In any case, get it evaluated before any more blood might be lost!