it's an aphthous stomatitis from what i can find in google images.
if its white in color and nothing is oozing maybe its just a canker (sp) sore. if it is above the tooth, where the tip of the root would be it could be an absess. when the tooth becomes infected the infection can come out the tip of the root, and drain through the gums. if you are not sure, do have a dentist look at it.