Upset stomach is very common and can be caused by several different reasons and leave you feeling gassy, constipated, nauseous, burning in your stomach or belly pain. Also known as indigestion, it is not a disease, but some symptoms you experience, including abdominal pain and a feeling of fullness soon after you start eating. Although it is common, each person may experience indigestion in a slightly different way. Some symptoms may be felt occasionally or as often as daily. Some times indigestion may be a symptom of another digestive disease. Indigestion that isn’t caused by an underlying disease may be eased with lifestyle changes and medication.