You had posted this question in another thread - I answered there but will repeat here:
I'm writing this with the caution that this NOT necessarily what your problem is. But I had something extremely similar - and in the same area.
I had a lump inside my mouth by my lower lip. It seemed to change size and times when I looked at it, it appeared swollen and even whitish.
I showed it to my dentist and said not to worry, most likely it was a blocked mucosal gland. Another souvenir of radiation and dry mouth. He said it never really goes away - has to be removed.
He sent me to an oral surgeon who agreed. He then numbed up the area, removed the lump, put a stitch in, and sent me home. He sent the tissue for analysis and came back again as nothing more that a blocked mucosal gland. Took the stitch out a week later and never been bothered again.
I'd suggest that since your dentist seems to be experienced at the effects of radiation, show him what's happening. But do it now. It may be nothing more that what I detailed above.
Please let me know if there's any other info you need - be glad to give you more detail.
Good luck - let us know how this works out. It may be nothing more than an overly disturbing annoyance.
- Jeff