Possibly and most common reason could be cheek biting. But you already give history of it being present for three weeks..so I suggest to consult n get an x-ray done to see any underlying cause. As you don't have any swelling or other symptoms except pain, it can be treated with least discomfort at this stage than waiting for severe symptoms to arise.
It may be because of cheek biting
Just wait and watch for 3 days and take some pain killers
Elaborate your symptoms like size of lump , it's colour if visible, any pus discharge or not
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