There are many reasons why menstruation does not occur, for example, the following: 1, more common is due to changes in the level of hormones in the body of some women, in the pre-menstrual period will appear pain in the belly, pay attention to observe whether the late menstruation will appear, and is due to pregnancy and the future of menstruation and pain in the belly; 2, due to acute pelvic inflammatory disease in women resulting in pelvic congestion, edema, and even pelvic The pain in the belly can occur due to acute pelvic inflammatory disease, but often accompanied by abnormal leucorrhea, gynecological examination or ultrasound examination can be further clarified; 3, the pain in the belly due to acute gastroenteritis, generally have nausea, vomiting, bloating and other gastrointestinal symptoms; 4, due to urinary system diseases, such as urinary tract infection, stones, etc., may appear pain.