Its called an oral ulceration, or mouth ulcer. It can be from a sharp tooth, biting the tongue, having braces or sometimes can be a bacterial sore. It generally goes away with time and can be aided...
Boxers as they age tend to get these benign tumors around their head and mouth. Some can grow in their mouth on gums or in the throat. Most of these you need not worry about, but I suggest you have...