Why is my stool loose when my stomach is upset?

Irritable bowel ? Bloating, irregular stools, and fullness may reflect diet (check your fiber intake), alterations in the population of healthy gut flora we each carry, food intolerances, maldigestion due to problems of the small intestine, pancreas, or biliary tree, changes in GI motility (how well your GI tract mixes and moves food along), various infections, and even irritable bowel syndrome. Testing is easy.