Pain in the corner of the belly is caused by constipation, proctitis, appendicitis, and other factors. When the pain in the corner of the belly occurs on the lower left side and is accompanied by difficulty in defecation, it is mostly caused by constipation. When constipation occurs, the bowel movement is slow, so the time to pass feces is relatively longer. When stool is stored in the sigmoid colon and rectum, toxic substances are released and the mucous membrane of the intestinal wall is irritated, thus causing pain in the lower left side of the abdomen. The rectum is located in the lower left abdomen of the body, and when inflammation occurs in the rectal area, the pain in the lower left belly corner is also accompanied by inflammatory factors that invade the smooth muscle of the rectum. Some people have pain in the right lower abdomen, and the pain is metastatic and more intense in nature, so they need to be alert to appendicitis. Acute appendicitis is more dangerous and can lead to fever, nausea, vomiting and other symptoms, which need to be treated as soon as possible and if necessary, appendectomy should be performed.