Our expert says:
It sounds very much as if your cat has an abscess, and you must get him/her to the vet as soon as possible - unless the swelling has really gone down. Even then, I'd take my cat to the vet just to be sure. An untreated abscess can poison the whole body and be fatal.
Basically, the abscess must be opened and allowed to drain. Sometimes the abscess ruptures by itself, but often the vet will lance the swelling, and expel the pus.
Sometimes the abscess closes over again and more swelling occurs - this mustn't be allowed to happen, and if it does you must go back to the vet.
You can also post this question to our CyberVet expert, but I think he'll have similar advice.
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