Did the vet give you any idea of what it might be?
A couple of things crossed my mind, one is feline acne, but that usually causes little black cruddy itchy bits (looks bit like flea dirt), although it can cause swelling/redness too. The other is eosinophilic granuloma - good name huh? otherwise known as rodent ulcer - causes are not really known, but suggestions include infection, food allergy and even irritation from their rough tongues (can't see that meself - I mean all cats have rough tongues and don't get EG!)
Its affecting her appetite then? - so must be sore, poor little soul - hope the anti-inflammatory and antibiotics work soon.