I have experienced the same sort of mouth symptoms you have. It got so bad that I went through the whole biopsy, CT/PET scan routine again to see if I had a recurrence. They assured me I did not, but that the biopsied pieces of tissue we sent in showed "radiation changes". I am 2 years out from my radiation therapy, and have noticed a couple of things that may play into the pain. I have some scar tissue at the back of my tongue that is very sore whenever I talk or yawn or otherwise move my tongue. I have also noticed that the numb places on my neck and ear from the surgery have become enervated again, I can feel sensation on this side my neck exactly as I can on the other side. It is possible that the tightening in the scarred area of the tongue and the renewed sensation in the area have combined to give yet another gift of oral cancer!
On the up side, I have lost 45 pounds.
You don't feel much like eating when it hurts to do so.
Yoghurt and soft boiled eggs are easiest to eat, because they coat the sore areas and give a little protection.
If you want the names of the products I use to reduce the mouth pain, rinses and ointments and sprays, e-mail me and I will pass that info on.

Live well,
Andrea