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.

Hi..could be because of cheek biting. Apply pansoral gel thrice daily on the affected area..

Get a check up as well as an X ray done to rule out any dental source of infection. That may help in the diagnosis and treatment planning

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