This is Dr. David
it sounds like you have a sinus or mucosal cyst or abscess which could contain an infection.
most likely that cheek bone injury 25 years ago caused this abnormal fluid conduit or fistula which is causing this fluid pocket to accumulate under your cheek.
you should see an ENT ear nose throat doctor about this and see if you can get it repaired by surgery.
or else, in the future, you run the risk of an infection getting into the area causing worse infection problems.
I'm glad your GP doctor started you on antibiotics and that seems like it is helping now.
let me know if you have other questions.
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