You may have a mouth ulcer if you have: A painful sore or sores inside your mouth, on the tongue, soft palate (the back portion of the roof of your mouth), or inside your cheeks; Sores in your mouth that are round, white, or grey in colour, with a red edge or border; In severe mouth ulcer attacks, you may also experience: Fever; Physical sluggishness; Swollen lymph nodes; How are mouth ulcers treated? Pain from a mouth ulcer generally lessens in a few days and the sores usually heal without ...