You have mouth sores developing. It's a side effect from the cancer treatment, chemo or radiation. Let your dr. know immediately so you can get a prescription for something they call Magic Mouthwash. It has lidocaine in it and you swish it around your mouth and hold it in for a minute or two. Watch what you eat, nothing spicy and stay away from alcohol, coffee and carbonated beverages. It may take one week to get over it. I have had it so bad that it hurt to swallow. Don't let it spread, call your dr. as soon as possible.
Good Luck!