Thrush maybe? I'd still get that checked out though. Seems like a long time for radiation sores but that would depend on the dose that that particular area of the tongue received. My doctors are just the opposite - I have already had 1 tongue excisional biopsy -turned out the culprit was my Water Pik -had it dialed in too hard and it irritated the tongue. The biopsy site still has some pain 4 years later...