It's just a canker sore. Gargle with salt water few times a day,it'll take a few days to a week to heal.

I’d gargle with warm water mixed with sea salt. I get ulcers quite often and I use bonjela when they’re either large, very painful or last a long time. If it continues to develop or becomes a serious issue with eating/drinking, there’s no harm in seeing the GP for some help/advice.