Not sure about cats, but if this were our dog, we'd suspect something tick-borne. Does this kitty get outside at all? Rotating lameness, painful joints, neurological symptoms, depression, lethargy, that icky feeling...that can all point to Lyme's, anaplasmosis, babesiosis, Rocky Mountain Spotted Fever or ehrlichiosis in my area. Not sure where you're located, but that might be something to discuss with kitty's vet.
Our kitty experts may have other suggestions.
I hope Bella recovers soon!