Hi，it might be mouth sores, it often go away in 10 to 14 days, even if you do not do anything. The following steps can make you feel better:
Avoid hot beverages and foods, spicy and salty foods, and citrus.
Gargle with salt water or cool water.
Eat fruit-flavored ice pops. This is helpful if you have a mouth burn.
Take pain relievers like acetaminophen.