With very few exceptions, the usual cause of a sudden swelling in a localized area is a blockage of fluid drainage (as in the case of the person who had a stone blocking a salivary duct, or due to a leakage of lymph fluid or blood into the area), or an infection. Usually it is an infection. These are often, but not always, so or painful, esp. when touched.

Depending on what is causing the infection and what sensitive structures it may or may not be near, it could be serious, and could become more serious very quickly. Therefore it is worth a trip to the doctor's office or clinic, soon, to be safe. I do not recommend waiting this one out or treating it yourself until it has been properly diagnosed. Good luck!