The pain in the left side of the abdomen after lunch every day may be due to inflammation of the colorectum, colorectal tumors, and inflammation of the left side of the reproductive system. 1. Inflammation of the colorectum: various external and internal causes lead to inflammation of the colorectum, which stimulates the gastrointestinal tract after eating and produces pain symptoms, and may be accompanied by diarrhea, nausea and vomiting and other gastrointestinal symptoms. 2. Colorectal tumor: Especially for the old people, after eating, gastrointestinal peristalsis stimulates the tumor in the colorectal area, resulting in pain in the left side of the abdomen, which may be accompanied by changes in defecation habits and stool characteristics. 3. Inflammation of the left side of the reproductive system annex: If it is a woman, it is necessary to consider the inflammation of the left side of the reproductive system annex, such as left salpingitis, left pelvic inflammation, etc. It is possible to do the left annex and pelvic inflammation. Ultrasound of the left adnexa and pelvis can be done to clarify. The causes of left-sided small abdominal pain are numerous and complex, and can involve lesions in multiple systems, which must be taken seriously. Attention to small abdominal pain in the absence of a clear cause before not blindly use analgesic drugs to avoid masking the condition, it is recommended to go to the local regular hospital for careful examination, and treatment under the guidance of the doctor.