A small bag under the jaw may be insect bite dermatitis, or folliculitis, lymphadenitis of the jaw, and so on. 1. Insect bite dermatitis: When the skin in the jaw area is bitten by mosquitoes, the venom secreted by the mosquitoes penetrates into the skin, which may lead to a small bag under the jaw, and may be accompanied by itching, pain and other symptoms. 2. Folliculitis: When the hair follicles and the surrounding tissues in the jaw area are infected by bacteria or fungi, folliculitis is induced, which may lead to a small bag under the jaw, and may be accompanied by local itching, pain and other symptoms. 3. Mandibular lymphadenitis: the causes of this disease are many, and it is considered that it may be related to upper respiratory tract infections, infectious diseases of the head and face, autoimmune diseases, etc., which leads to the enlargement of mandibular lymph nodes, resulting in a small bag under the jaw, which may be accompanied by pressure and pain. There is a small bag under the jaw, may also be other diseases, if accompanied by other obvious discomfort, it is recommended to consult a doctor as soon as possible to clarify the diagnosis, give targeted treatment.