My lower left of back will start to hurt usually right before my period. Sometimes right after also. It gets so bad sometimes that all I can do is sit or lay down. After I sit or lay down for awhile it lets up. I will take advil, mortin, etc. for the pain but does not help it. Lately it has moved to the front, its more of a achey feeling but sometimes it can double me over.
It's physiological. Uterine endometrial contraction causes lower abdominal pain. The uterine plexus is connected behind to lower vertebral plexus, which causes low back pain during and after menstruation.