Yes your hips, thighs and lower back are used the different elevation of the walking boot and a normal shoe. It will go away soon.
Sounds like you need proper shoes - consider orthotic inserts or shoes. Google or Bing "Foot Smart" catalog of choices. once I went off a boot, I began to have back alignment pains and problems until I addressed it with shoe inserts and physical therapy. Good Luck.
I would think that it is most likely not normal. Did the doctor say it was time to take the "boot' off? Does the doctor who is treating you know about your "excruciating" pain? My guess was going to be that you are "stiff" from not moving around as much as a result of the ankle injury, but not if the pain is described as "excruciating". Definitely let the doctor know about this. good Luck,
shinty