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Hello. Thanks for the question.
The main concern I have for swelling under the jaw and of the mouth, is a tooth root abscess. In cats, we can see an abscess lead to swelling under the jaw. This is because their tooth roots are quite long.
The diagnosis is usually made with dental x-rays. In really severe cases, we can see gingival involvement on oral exam. Most cats will require anesthesia and dental x-rays, though. Once diagnosed, the infected tooth or teeth can be removed. This is curative.
I hope this helps, let me know if you have any other questions.