I have a similar pain that comes and goes. As a survivor it has worried me. I think about all the bad things it can be, but scans don't show anything there. A Panorex film would show if there was a radiation induced osteosarcoma, which looks like soap bubbles on the film. Make sure the dentist knows what he is looking at. It isn't a common thing to see. So the worst possible thing may be eliminated from your differential diagnosis. Ditto cancerous involvement of the bone in that area would also show up as a dark radiolucency on the film. A CT of that area would disclose any soft tissue related mass, and just to be safe you could eliminate that possibility with the scan

Now for my own conclusion after chasing this for a year. Trismus is an issue. Radiation has formed scar tissue in the muscles of your face and which control your jaw movements. On the facial side this is primarily the masseter muscle, on the inside of the mandible, this is the internal and external pterigoids. My pain occurs (to me) on the mandible itself, but in reality it is the insertion /attachment points of these muscles which are very sore. They can become that way from a lot of chewing, a great deal of talking (which I am prone to in spite of my dry mouth) and in particular from clenching my teeth which I do unconsciously. I have bruxed/ground my teeth all my life in sports, and when I am nervous (too much of the time now) and I even do it during my sleep. I wake up in the morning with a really sore mandible, and I believe it is because of this.

Muscle fatigue and overwork brought on by tension will cause this kind of pain. I offer this as food for thought and not necessarily a diagnosis that applies to you.