This could be the result of gastro-esophageal reflux disease (GERD) whereby stomach content goes back upward into the esophagus, usually because the muscle ring (lower esophageal sphincter) that separates the esophagus from the stomach and works to prevent backward movement of stomach content, is incompetent (loose). Another possibility is hiatal hernia, which is protrusion of a part of the upper segment of the stomach into the chest through a defect in the diaphragm, usually resulting from unusually elevated pressure in the abdomen. Hiatal hernia is also a cause of acid reflux.