Pain on the left side of the navel, mainly in the middle abdomen on the left side, may be caused by the following reasons: 1. It may be a gastrointestinal problem, such as acute or chronic enteritis, gastrointestinal dysfunction, intestinal spasm, intestinal obstruction, intestinal adhesions, etc. It may also be a local intestinal polyp or tumor, which can cause pain. 2. It may be a disease of the ureter, such as ureteral stones or urinary tract infection. In the case of ureteral stones, the pain is sudden and intense, accompanied by pain in the left kidney area and percussion pain, and some patients also have symptoms of blood in the urine. In case of urinary tract infection, it is usually accompanied by frequent, urgent and painful urination.