If it is in the lip, instead of on the chin, check online for photographs of eusinophilic granuloma, and see if that looks the same. But those are usually bigger than you seem to be describing.
is this sore black? if so its likely feline acne and nothing to do with diet.....
If you feed in plastic bowls.. stop and be sure to use ceramic or stainless steel ones and wash with soap and water daily.... plastic harbors spores w/ bacteria.