Along with what HRP mentioned, I'd wonder about the possibility of Lupus (Systemic Lupus Erythematosus in particular). It's rare in cats but the symptoms are rotating lameness, lethargy, anorexia and sometimes skin lesions. Unfortunately there isn't an actual test for it - diagnosis is based on the exclusion of other possible causes. Here's some more info:
http://www.petwave.com/Cats/Health/I.../Overview.aspx
Crossing my fingers that the vets can get to the bottom of this and that Bella makes a full recovery! Do let us know what you find out.....