I've had bumps that were just due to some irritation post-radiation. You really aren't that far out from radiation so that certainly seems to me to be a possibility but, as David says, it's a good idea to have it looked at. Can your MO do a scope of your throat? If, as you said, this is on the base of your tongue, not the oral tongue, I agree with David the MO might not be the person to give you the final all clear. But I'm sure she or he will send you on to the right person if necessary.

Nelie