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if you have braces that could cause it i know when i had braces i had them alot, best thing i found was put wax over the braces and use carmex or some sort of coldsore medicine
also they are a form of herpies, a very weak form but a form none the less, they can be caused by a lot of sucking (kissing). or even biting the insides of your cheeks.
at least thats what i hear.
hope that helps
As you're only 13, I'll refrain from giving the obvious answer!!