Pain and swelling in the back gum around the last molar are most commonly due to a problem with the Wisdom tooth impaction or a local gum infection (pericoronitis), where food and bacteria get trapped around a partially erupted tooth, causing pain, swelling, and sometimes difficulty in opening the mouth. Initial care includes maintaining good oral hygiene, gentle brushing, and rinsing with warm salt water, and you can take a mild pain reliever if needed, but this usually requires a dental check-up. A dentist may prescribe medicines to control infection or recommend cleaning or removal of the tooth if it is the cause. Seek care promptly if swelling increases, you develop a fever, or have difficulty swallowing or opening your mouth.