Swelling of the flesh behind the last tooth may be the result of incomplete eruption of wisdom teeth forming a blind pocket, with pericoronitis of wisdom teeth, or periapical inflammation of the second molar, with pus spreading to the posterior region of the molar, as follows. 1. Wisdom tooth pericoronitis: the surface of the wisdom tooth is covered with a gingival flap to form a blind pocket, food and bacteria accumulate in the blind pocket, resulting in inflammation of the soft tissues, swelling, pain, pus overflow and other symptoms. 2. Second molar periapical inflammation: inflammation of the apical region of the second molar may appear pus, pus spread to the back of the molar to form swelling, the second molar may have symptoms of knocking pain, occlusal pain. There may be other reasons for the swelling behind the last tooth, so it is recommended to go to the hospital in time to clarify the cause of the disease and then carry out targeted treatment.