Is it just a bump? Or can you see a center to it, like with a zit or blackhead.
Since she's had it for a while in the exact same spot, it sounds less like acne - but it's still possible that it could be. Asking the vet to look at it the next time she's at the vet would be a good idea.
To help stop/prevent acne around cats mouths they need to be fed from dishes that can be sterilized. Glass, higher quality glazed ceramic or porcelain, or better yet stainless steel. Wash after every meal.
Washing a cat's face after they eat helps, too, since some cats get food and the oils from it on their faces and can't get them fully cleaned off.