What would be causing a very sore coccyx during a particularly difficult menstrual cycle?

Usually it is due to a spasm of the gluteal muscles, assuming there is no history of injury. It is better with walking and may be worsened by sitting. Generally it can be relieved by painkillers, using a donut cushion,and hot baths . However if it persists, along with extremely painful periods, it might be advisable to have the doctor check you out for a condition called endometriosis.

You fell or sat in one place too long??