Hello, Not in every patient but Mucinex in some patients does cause diarrhea and digestive issues like stomach pain, nausea, vomiting, dizziness, etc. Take anti diarrheal medicines and adequate amount of water and fluids. You are diagnosed with bleeding ulcers. Bleeding ulcers are triggered due to some medicines, infections, etc. You are suggested to get clinically examined by a GP, get endoscopy, required blood tests and imaging tests done. Black stool are indicative of bleeding ulcers. Profuse bleeding from ulcer can be life threatening. Ulcers are treated with PPIs or H2 blockers. If required, your treating doctor will treat you by injecting medicines, cauterization, etc. Do not neglect yourself, get examined & evaluated. Get treated at the earliest. Take care. Hope I have answered your question. Let me know if I can assist you further. Regards, Dr. Nupur K., General & Family Physician