Stomach discomfort when lying down may be due to poor diet, gastritis or ulcer, gastroesophageal reflux disease (GERD), gastrointestinal tumors, and so on. 1. Dietary irregularities: stomach pain, bloating and other stomach discomfort symptoms may occur when lying down after eating too much or eating unclean food. 2. Gastritis or ulcer: if the patient has a history of gastritis, the gastric mucosa will be irritated when lying down and symptoms of gastric discomfort will occur. 3. Gastroesophageal Reflux Disease (GERD): when the patient lies down, due to the change of body position, gastric acid reflux occurs, which leads to the symptoms of stomach discomfort. 4. Tumor of digestive tract: various tumors of digestive tract may cause stomach discomfort because of pulling the surrounding tissues when lying down. There are many other reasons for stomach discomfort when lying down, patients should consult the doctor in time, improve the relevant examination to clarify the cause of the disease, and treat under the regular guidance of the doctor.