If you have stomach cramps every time you have your period and have a bowel movement, clinically it may be caused by endometriosis. For example, ultrasound examination of the pelvic cavity, combined with clinical history of infertility, dysmenorrhea, or clots during menstruation, may be useful for a comprehensive analysis and, if necessary, a blood test to see if CA125 is elevated. If CA125 is mildly elevated, it may be clinically caused by endometriosis, and appropriate medication is recommended. In addition, stomach cramps during menstruation may also be caused by prostaglandins produced during the shedding of the endometrium, causing contraction of the smooth muscles of the intestines and causing cramps.