An upset stomach refers to a condition where an individual experiences discomfort, pain or nausea in their digestive system. This can be caused by various factors such as overeating, food poisoning, stress, anxiety, viral infections, bacterial infections and consumption of certain medications. The symptoms may include bloating, gas, heartburn, indigestion, diarrhea or constipation. An upset stomach is generally not a serious condition but can be uncomfortable and inconvenient for the individual affected.