Did your vet tell you that RODENT ULCER can come back? Is it possible that it is a reaccurance of it? Kylie has had SEVERAL flareups of the R.U. and I usually clean it with peroxide and put hydrocortisone cream on it. If it gets too bad I Take her to the vet. She is now on a half a pill a week of an antibiotic (maybe). She has also had the skin form. She gets large open scabs, around her neck. Therefore the vet said since she has such sensitive skin to forget about her collar. Whenever she wore one it irritated it.
One of my friends has Kylie's sister, and she had this under her chin also and she squeezed it and said it was like a very large blackhead or whitehead. Kylie has never had that, It is just flaky, that you can flake off with your finger.

Good Luck!!