This sounds very reminiscent of my RB Lola. She was a stray I found as a kitten, and I had her for about a month or so, when I noticed this bumpy rash all over her chin, and it was itchy cause she scratched all the time. I finally figured out she was allergic to her food dish!!! Of all things! SO try switching her food dish to one made out of another material.

Good luck!