Gastroenteritis (stomach flu) is a viral condition that causes diarrhea and vomiting. Iron poisoning causes abdominal pain, vomiting, diarrhea, lethargy, and dehydration. Colon cancer is often asymptomatic, but it can cause constipation, bowel obstruction, bloody stool and more. Peptic ulcers, sores in the lining of the stomach or upper intestine, cause abdominal pain, gas, and more.