Mike,
When my mouth started doing the exact same thing yours is doing, at the same point in treatment..........I called my husband SO SURE that my cancer was back. I called my radiation nurse, begging her for an appointment. She told me it was the beginnings of mucositis and that it would only get worse. I so recall that conversation, I didn't have a cell phone at the time, was driving to the store, and stopped at a payphone when I noticed the stuff in my mouth.
Take a deep breath because it sounds like you are in the "normal" range for radiation treatment.
Minnie