I read this thread after searching on Google. Two nights ago, my cat "Radley" displayed the same trouble. She is 7 years old and has also always been an indoor cat, but she suddenly had trouble walking and the Vet and I are puzzled. I initially thought she had gotten hurt somehow, but the vet observed her all day yesterday and said she is displaying what he called "Shifting Lameness" which is unusual and her x-rays came out normal.
She did go to the Vet 2-3 months ago because she was having allergies, and the Vet gave her a steroid shot. The Vet said Diabetes was a possible side effect of that shot, and I am wondering if this may be a result of that as well. I am supposed to get the lab results on her blood and neurological tests today. I would appreciate if some of these posters with similar problems could keep posting with whatever they find, and I will do the same.