If anal distension occurs when menstruation occurs, it is often clinically caused by the following reasons: 1. the uterus is in a retroflexed and retroverted position, the pelvic cavity is in a state of congestion during menstruation, the uterus will stimulate the rectum, so it will lead to anal pain. 2. caused by endometriosis, if there is an endometriosis lesion in the rectum or in the rectal sockets of the uterus, when menstruation occurs The ectopic lesion will also bleed, so there will be localized anal cramping and pain, and some people will even have blood in the stool. Further clinical examination, including ultrasound, gynecological examination and anal examination, is needed to find out whether there are local foci of endometriosis, and then treatment should be given.