It sounds like a canker sore. Here is what a canker sore is.
What are canker sores?
Canker sores are shallow, painful sores in your mouth. They are usually red or may sometimes have a white coating over them. You might get them on the inside of your lips, the insides of your cheeks, the base of your gums or under your tongue. Canker sores are different from fever blisters, which usually are on the outside of your lips or the corners of your mouth.
Anyone can get canker sores, but women people in their teens and 20s get them more often. Canker sores may run in families, but they aren't contagious. Doctors don't know what causes canker sores, but they may be triggered by stress, poor nutrition, food allergies and menstrual periods.