Small abdominal distension right below the belly button may be related to menstrual period, pelvic inflammatory disease, ulcerative colitis and so on. 1. Dysmenorrhea: usually when a woman is menstruating, there is pain in the lower abdomen, especially in the lower abdomen. 2. Pelvic inflammatory disease: not paying attention to personal hygiene in general or during menstruation, or having sex during menstruation may lead to pelvic infections and show bloating and pain in the lower abdomen. 3. Ulcerative colitis: intestinal immune imbalance caused by the interaction of environmental, genetic and intestinal micro-ecological factors, causing intestinal mucosal breakage and localized ulcerative surfaces, which may also appear in the left lower abdomen or the lower abdomen with hidden pain. In addition to the above, it can also be seen in other causes, the occurrence of small abdominal distension and pain, it is recommended to promptly consult a doctor, follow the medical advice for further examination, with the help of the doctor to clarify the cause, and actively cooperate with the doctor to deal with or treat.