Hey, I had that once. I used, per my a booklet, 1/2 baking soda, 1/2 salt, gargle 4 x a day after each meal and at bedtime. It worked.
I had someone tell me that baking soda toothpaste worked, and since i used it already, i didn't have to do anything new... but i never had a mouth sore! so i would try that...good luck....hope it helps!