Hi Minnie,
first off thanks for your info about the jaw and bone graft!
here are some of my opinions (not facts):

I've found - prior to cancer - that a lot of sugar, or salty things, would give me canker sores, or what I think of as pre-canker sores - an area that was sensitive, red and sore. (My mom says she can't drink diet sodas because the artificial sweetners give her canker sores.)

So, I think some things, in close or constant contact with the mouth tissue are irritating the area. Such as throughout a long car ride. Seems to make some sense. Add onto that with cancer and its various treatments the mouth area is, as Bob/jetage hobo says always prone to something!

I'd try rinsing with water more often to try to get food particles, etc away. Also one of my doctors likes to recommend plain old salt water as a general way to cleanse the mouth and keep off bacteria.

Maybe its thrush or fungus. I had/have the start of cold sores at the corners of my mouth and have used a nystatin creme. The creme might not work inside your mouth but during radiation I had a nystatin rinse to help more sores. And prior to cancer (but during pre-cancer leukoplakia stage) my ENT gave me carafate for canker sores that worked well. You might want to ask your doctor about those.

Others in my general cancer support group have commented about how prone their are to little things - skin abrasions, cuts, etc. that seem to cause more trouble than in the pre-cancer days. They share the feelings that these minor issues are of course nothing like the threat of cancer, but are the lingering little, frustrating aspects!
And of course the constant fear of worrying whether some little symptom is a lurking cancer!

Hope it clears up soon!