Painful periods (or dysmenorrhea) is the hallmark of “endometriosis”. It is particularly felt in the “pit of your stomach” in the days leading up to your period.
In the autonomic denervation view it arises from injuries to pelvic nerves as they enter the uterus at the junction of the cervix and the body of the uterus. Abnormal nerves in the lower part of the uterus gives rise to this specific symptom. Effective treatment will require reductions in pelvic blood flow with progestagens, oral contraceptive preparations and, most effectively, GnRH agonists (Lupron, Synalar, Prostap, etc.)