The phrase "upset tummy" is a common term used to describe an uncomfortable feeling in the stomach, often accompanied by symptoms such as nausea, vomiting or diarrhea. It can be caused by various factors including food poisoning, viral infections, stress, anxiety or overeating. The phrase is a colloquial way of referring to gastroenteritis which is an inflammation of the stomach and intestines that results from bacteria, virus or parasites entering the digestive system.