The term heartburn describes a burning sensation in the chest beneath the breastbone. It is caused by a backward flow of acid-containing stomach contents, which irritate and burn the tissue lining the esophagus. This tube connecting the mouth and stomach has a valve to prevent this backward flow. Heartburn occurs if the valve leaks, such as after eating and lying down. Other symptoms may include a sour taste in your mouth, burping, bloating, nausea and regurgitation. Heartburn is common at any age.