It may be a type of fungal infection.. you should see your dentist for further evaluation. In the mean time you can try to gargle with 1/2 peroxide 1/2 water and/or salt water. It could even be canker sores, but without seeing them is hard to say, so do see your doctor for proper treatment.
Hope this helps, If you need anything else please feel free to ask. If you found this helpful don’t forget to click the *accept* button!
