Probably this. Our cat had one like that recently. Vet cleaned it out and put him on antibiotics for about 10 days. Said it was likely from a puncture wound from a fight with another cat.
Agreed have vet open it up and get some antibiotics
My golden had a lot so I ordered fishmox (amoxicillin) and would open them and give meds myself but that was after dealing with a vet I wouldn't have done that before the vet trip
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