cat is very skinny

I have a 17 year old female that has been losing weight over the last few months. She has always been a skinny cat so it only became a concern when I could feel every bone in her body. Let me say that other than her weight loss, she showed no symptoms of any health issues. She would eat like a horse and and use the litter box normally. So I took her to my vet where he did some blood work, thinking thyroid. The thyroid came back normal and the only issue shown was an elevated white blood cell count. She weighed in at a little over 4 pounds. I brought her back on 1-20-15 and he gave her an injection of prednisone and started her on clavamox. Here's where the problem starts. AS SOON AS I got her back in the truck I noticed a difference. She hates riding so she sits at the cage door and meows constantly. When I put her on the seat she laid down and didnt say a word all the way home. Since that moment she became extremely lethargic, doesn't like to eat, is not getting up to go pee, very very weak meow, can barely stand up and gingerly walks but staggers like she's drunk.

So I guess my question is is this an allergic reaction to the meds that she was given or is something else going on? I have have stopped the clavamox. Not sure if I want to go back to the same vet or find a new one.

Any help would be appreciated.

It's very possibly a reaction to the meds.  I would get a second opinion if I were you.  I know Clavamox can make kitties nauseous and then they don't want to eat. 

Thank you Steph. That was kind of my thoughts. What im worried about is the vet trying to backtrack from this because I called him on friday and told him what was going on and he said it was fine give her the clavamox for a couple more days and call us back. When I called yesterday he said I could stop the meds " if I wanted".

I would def get a second opinion, did they only take bloodwork?

Yes. They only did bloodwork.