The pain in the stomach during menstruation is caused by the following reasons: 1. primary dysmenorrhea, which is often caused by the shedding of the endometrium, resulting in the secretion of a large amount of prostaglandins, which can cause abnormal contraction of the smooth muscle of the uterus, resulting in pain in the lower abdomen during menstruation. 2. secondary dysmenorrhea, which is often secondary to a disease or inflammation, such as pelvic inflammatory disease, which can cause dysmenorrhea, and endometriosis, which can cause relatively severe dysmenorrhea. 3. 3.Uterine malformation can cause uncoordinated contraction of the uterus during menstruation, which may also cause dysmenorrhea in clinical practice. Therefore, once dysmenorrhea occurs, it is recommended to do a detailed examination and then give targeted treatment according to the cause.