What is acid reflux (GERD)?
When the ring of muscles, called the lower esophageal sphincter, weaken over time, the valve may not close all the way or open too frequently, allowing acid to reflux into the esophagus. Individuals can also develop hiatal hernias, which can cause chronic inflammation and pain in the esophagus.
http://www.good4utah.com/news/local-news/unexplained-stomach-pain-could-it-be-acid-reflux