Giardiasis is an infection of the small intestine causing diarrhea, gas, bloating, nausea and stomach cramps. Inflammatory bowel disease. Inflammatory bowel disease causes swelling and symptoms such as cramping, chronic diarrhea, and bleeding. Viral gastroenteritis. Gastroenteritis (stomach flu) is a viral condition that causes diarrhea and vomiting. Medication reaction or side-effect Medication side effects include nausea, vomiting, stomach upset, weakness, dizziness, seizures, and more.