What causes mouth sore?!
Question:
What causes mouth sore?
Answers:
There are different kinds of mouth sore.
The "canker" sore, or apthous ulcer is often caused by stress, acidic foods (such as lemon), or Bechet's disease (which I'm sure you don't have). Nobody really knows why they happen, but stress seems to be a big part.
The "cold" sore is a herpes virus that will never go away and will keep coming back, especially when you are tired or ill.
Either way, get an over the counter medicine to rub on it. There really isnt very much you can do except wait for it to go away.