The stomach produces hydrochloric acid to break down food. If the body has more food to digest, this means more acid which will irritate the lining of the stomach. The stomach remains swollen when there are large quantities of food in it. The muscle located between the esophagus and the stomach won’t close properly, which leads to food and stomach juices to rise back up into the esophagus. That’s why heartburn has a burning sensation, according to WebMD.