On the first day of menstruation, the anus is bloated, mainly due to the following reasons: 1, because the pelvis will be in a state of congestion during menstruation, coupled with the uterus is in the posterior flexion and tilt position, the endometrium is shed during menstruation, the uterine cavity will have menstrual blood, which stimulates the posterior peritoneum, stimulating the rectum, so it will lead to anal bloating, we recommend lying on your stomach and resting on your stomach to alleviate the corresponding symptoms. 2, because the There are foci of endometriosis in the rectum, when menstruation is bleeding caused by the shedding of the endometrium, and the foci of endometriosis will also be bleeding with the shedding of the endometrium and bleeding, so it will lead to localized swelling in the anus or even more serious pain, it is recommended to go to the hospital for examination in a timely manner after menstruation is cleaned up to give the appropriate treatment.