Exotic Shorthair Urgent Health Question

I got my new kitten - Pudding - a few weeks ago. She is about 4 months old. Lately she hasn't been herself. She has lost some balance and control of her back legs. They are very weak and tend to slip out from under her when she walks. She has been using the litter box and eating and drinking like normal. When I noticed her limp legs, I took her to the emergency vet, but he couldn't tell us much.

All her vitals were fine, no fever. He felt along her spine, and squeezed different parts of her body, but wasn't able to isolate a trouble area.

He did an x-ray to see if maybe something was wrong with her spine causing her to lose balance. Her x-ray was normal and he couldn't find any fractures or anything abnormal about it. 

He then proceeded to tell us worst case scenario - she could have some rare disease and die.

He mentioned a number of tests we could do on her, but those can get very expensive. 

We opted to just give her antibiotics for a week or so to see if she improved. 

Can anyone please tell me what is wrong with my Puddin??

She seems happy, not in pain, just tired.

It could really be any number of things.  Has she had complete blood work done?  If not I would get that done to check all of her organ functions.  Yes complete diagnostics can get expensive when your talking things like MRIs but complete blood work, though not cheap doesn't fall in the category of very expensive diagnostics and can tell a lot.

thank you