Lower back locks up during period?


So I’ve had these scary episodes twice. It starts with the most severe period cramp, and then out of nowhere, my lower back locks up, severe pain, and my vision goes black. Both times, it has knocked me onto the floor.

Once I’m able to stagger into a standing position, the pain shoots all the way down both legs and makes them turn inward a little bit.

Usually it unlocks but the pain is still so severe and lasts the rest of the day. In the following days, it’s really stiff and sore.

It ALWAYS happens just during my period. My fiancé googled it and he thinks it could be endometriosis. I needed to go to the doctor but my insurance has not kicked in yet.

What could this be?