It sounds as if your new uro is advancing from moderate to advanced MBD. Primarily, you should take it to your vet for a blood calcium test. They can draw blood and send it to the lab for quick results. We usually get our lab results faxed back within 24 hours. They may recommend two calcium injections (a week apart) to try to stop the advancing disease. In the mean time, I suggest starting it on a regime of calcium syrup (calcium calcionate syrup ... AKA neo calglucon) to begin ASAP. You can find it at most major pharmacies as an over the counter item. If they don't readily stock it, they can easily order it for you.
The bump near the area of the spine and the fact that they're not using their hind quarters much ... not to mention that they can't upright themselves after falling on their backs is strong evidence of this. Most of the cases that have reached this stage are often difficult to turn around. But we just have to try ... don't we?
You should also check it's joints to see if they're stiff/rubbery, swollen and for any signs of disfigurement. Make sure it has fresh UVB lighting and that it's getting a good calcium supplement with vit-d3 which should be lightly sprinkled in it's salad and mixed in good. Be sure to include the darker greens such as collards, mustards and any other dark green calcium fortified veggies.
Hope this helps in stopping what sounds like advancing Metabolic Bone Disease which can more easily be controlled in it's earlier stages but seldom in it's advanced stages. I don't want to lead you on but you're going to have a fight on your hands with this dreaded disease by your described symptoms.
Keep us updated on how it goes and good luck,
John
