A bulge behind the ear may be caused by sebaceous cyst behind the ear, lymphadenitis or mastoiditis. 1. Sebaceous cysts: If the local pores behind the ear are blocked, the oil can not be discharged, and a long time accumulation of sebaceous cysts will be formed, resulting in the symptom of a bulge behind the ear. 2. Lymphadenitis behind the ear: If mumps, otitis externa and other local infections, involving the lymph nodes behind the ear, inflammation stimulation caused by the lymph nodes behind the ear, resulting in swelling, thus appearing behind the ear with bulging symptoms. 3. Mastoiditis: If the condition worsens and causes complications such as subperiosteal abscess of the mastoid process, it may manifest as a bulge behind the ear. If the bulge symptoms are not relieved for a long time or even aggravated, it is recommended to consult a doctor in time to find out the cause of the bulge behind the ear and treat it positively so as not to delay the condition.