Heartburn happens when the lining of the esophagus comes in contact with too much stomach juice, which can produce a burning pain and injure the esophagus. There is a valve that connects the end of the esophagus with the stomach, which normally functions to keep the stomach acid where it belongs--in the stomach. But in people with frequent heartburn, this valve relaxes too frequently, allowing the stomach acid to splash up into the esophagus.