Stomach pain once lying down may be caused by acute gastroenteritis, stomach tumor, acute pancreatitis and acute cholecystitis. 1. Acute gastroenteritis: If you suffer from acute gastroenteritis, the symptoms of stomach pain will appear. When the patient lies down, the symptoms will be more serious, accompanied by nausea, vomiting, diarrhea and other symptoms, it is recommended that patients with gastroenteritis do not lie down after eating, and seek medical attention in time. 2. Stomach tumor: With the enlargement of tumor, gastric mucosa is eroded by cancer cells, especially the tumor of gastric sinus, which leads to the obstruction of the pyloric valve after enlargement, resulting in gastric dilatation, nausea and vomiting, etc. The patient may have the symptom of stomach pain when lying down, and some of the patients may suffer from sudden severe abdominal pain, etc. These symptoms are recommended to be timely taken to the hospital, accompanied with nausea and vomiting and diarrhea. If these symptoms occur, it is recommended to go to the hospital in time. 3. Acute pancreatitis: Acute pancreatitis refers to the inflammation of the pancreas. When acute pancreatitis strikes, there is usually a sudden and persistent severe pain in the upper abdomen, accompanied by abdominal distension and nausea and vomiting. When the patient lies down flat, the symptoms of stomach pain will appear. 4. Acute cholecystitis: Acute cholecystitis mainly refers to acute inflammatory lesions of the gallbladder caused by obstruction of the gallbladder duct, bacterial infection, etc., and symptoms such as epigastric distension and pain and discomfort, accompanied by nausea and vomiting, loss of appetite and other symptoms will occur during the attack. When the patient lies down, the symptoms of stomach pain will appear. If the above symptoms recur or do not improve for a long time, it is necessary to go to a specialized hospital for examination and treatment.