Cat Medical inquiry

Question
I have an 11 years old female cat. A few weeks ago she slowly became sick and it got to the point where her 'backend' seems floppy (for lack of a better word. In other words, she can only walk a couple steps and her backend gives out and she flops to the ground. Also, she has lesions in her mouth which have an unpleasant odor. She does take a few bites of food and drinks, but not excessively. We have had her to a veterinarian and he diagnosed her with kidney failure. However, I've been researching this on the computer and her symptoms aren't compatible with his diagnosis. She's not going to the bathroom excessively or in inappropriate places, for instance. She is on antibiotic and it has seemed to help somewhat but she still seems extremely tired and she just can't get around very well. I'm very worried about her but I also realize that she is probably close to death. I'm just looking for answers. Thank you.
I also need to add that this 'sickness' appeared suddenly the day after she had been outside for most of the day. It hasn't been something that we've noticed in her previously, which also adds to my doubt in the kidney failure diagnosis.  

Answer
I cannot take the place for what your vet said, so I urge you to go with what they said. They are professionals. It sounds to me like she might have gotten into some poison while she was out. Did your vet test for anything like that? I believe poison can cause kidney failure. Well, I really hope everything works out. I'll pray for your kitty. Have a good night hon.