Yikes! Do you normally take any anti spasmodic medication like cyclobenzaprine or baclofen? I found the latter works better, doesn’t make me drowsy and less side effects such as dry mouth. That’s a lot of fusions! My neck is fused at same levels.

Reading your description "... until it unclenches." makes me wonder if the spinal issues I have in my back make my muscles around my hips clench in spasm. They literally make me feel like they'll throw me to the floor, and I can only hold on and until it lets loose. My cervical spinal issues cause muscle spasm, too, but not the same way. I can end up with cervicogenic headache.