What causes sore in the mouth? found it brushing teeth?

Canker Sores: Can be linked to problems with the body's immune defense system. The sores may occur after a mouth injury due to dental work, aggressive tooth cleaning, or biting the tongue or cheek. They usually heal without any treatment in about two weeks or so. In office treatment we use debacterol, a medication containing 50% sulfonated phenolics and 30% sulfuric acid. They heal in two days. Dr. Use only.

Sometimes Unknown: It can also be viral, hormonal, or an allergic reaction to some type of food or chemicals... Laser treatment can also help it heal faster.

Could also be: Besides the excellent answers the other dentist have given you, it is most important to also consider that this sore can be other things and it is important to know exactly where it is. Some of the other things it could be is a tooth or gum infection, a cancer, a traumatic injury, a burn from hot foods or drink, etc. Let the dentist take a look at your particular area to obtain the details.