Hello, Dark green stool has no concern with menses. If the same was started after the intake of Pepto Bismol, that could be in concern with the side effect of the drug. If that was not the case, it can be GI flora upset, intestine infection or others. Following guidelines can help you: - Drinking more liquids including boiled water, lemon juice, coconut water, oral rehydration salt - Prefer bland vegan diet - Pre and pro biotics as lacto bacillus, racecadrotil help great way - Some cases may need antibiotic as Metronidazole or others as per the choice of the treating physician. Take care. Hope I have answered your question. Let me know if I can assist you further. Regards, Dr. Bhagyesh V. Patel, General Surgeon