What's wrong with mortisha's chin?

I do not know how or when exactly this appeared but I do know it was over a long period of time (several weeks). She appears healthy aside from the obvious injury/absess. She's eating well and is bright eyed and active. I have a feeling it's some form of mouth rot but from all my research "mouth rot" usually apears inside the mouth not outside. Anybody have any thoughts as to what this could be and how to treat it?