Regurgitation occurs when food particles and liquid back up unexpectedly from the stomach into the esophagus and mouth. Regurgitation is caused by a weakness in the valve (lower esophageal sphincter) between the esophagus and the stomach that normally prevents food from moving back up from the stomach into the esophagus.
Regurgitation does not cause nausea or discomfort. Also, regurgitation is different than vomiting, because nausea and the awareness of soon becoming sick usually occur before vomiting. Regurgitation usually occurs without warning when a person is lying down or bending over.
WebMD Medical Reference from Healthwise