What Causes Excruciating Pain And Cramping During Periods?

I have excruiating pain during my period, often times it s every other month. Along with uterine cramping this new pain I m experiencing is like shooting, sharp pain near my tailbone or rectum. It is sharp, sudden, & so intense it takes me breath away & makes me cry out in pain. I would describe it as attacks because the pain comes on suddenly & shoots through me every 2 or 3 minutes for an hour or two then goes away. I cannot walk, sit, stand, lie down when it is happening, just hang onto whatever is nearby. The shooting sharp pain will wake me from a dead sleep. Ibuprofen does help, but only dulls it somewhat. It started between 6 months to 1 year ago. Please help, it is agonizing & it interrupts my daily life.