Whats wrong with my Cat?

It started with a slow walk, then taking two steps and resting. She has been diagnosed as being depressed and put on medication two years ago after all tests revealed nothing. We took her back to the vet, the best in Toronto, and had all the tests redone, blood and Xrays, two weeks ago, nothing was found again except a spur at the end of the spine before the tail. Since then she has lost all strength in the back legs, yet she can move them, and just drags herself around. We have to confine her to an area because she cannot get into her litter box, even though we have cut the entrance out. She is very overweight and has been put on a special Medi Cal diet plan, this is not on the list of recalled pet foods. She was never given any of the recalled stuff. We are all baffled, she is still with it, eats, pees, poohs and is bossie. Does anyone know what is wrong and have experienced this. We are slowly withdrawing her off the depression medication and she seems fine mentally.