Oh yes! I have suffered back ache! I have rheumatoid arthritis (had it since I was a babba) but the pain in my back while on LT has been so different from what I'm used to.
After losing about 3 stone it started. My spine was aching all day, and I had a sharp pain in and around the bottom of my back and around my bra strap. I went to the Docs, and she told me that my spine was adjusting. It had spent so long carrying so much weight that it had 'settled' Now that the weight had gone and the pressure had lessened, it had to redistribute and adjust.
It did adjust and the pain left. I also think exercise helped a lot.
BUT since losing 3 more stone and not be allowed to exercise because of swine flu, the pain has returned! I'm hoping it readjusts soon, and I'm back at the gym, so hopefully that will help.
Saying all that Sabriel, please go to the Docs and get it checked. Do not neglect it if you think it's your kidneys. Let us know how you get on.