Pain behind the left ear may be caused by acute middle ear mastoiditis, acute lymphadenitis, folliculitis and so on. 1. Acute middle ear mastoiditis: it is mainly an acute purulent inflammation of the mucous membrane of the mastoid air cavity of the left ear, which will cause left ear pain, redness and swelling behind the ear. 2. Acute lymphadenitis: acute purulent inflammation caused by bacterial infection of the lymph nodes behind the ear, which will result in swollen lymph nodes in the left ear and pain behind the ear. 3. Folliculitis: Inflammation caused by bacterial infection of the hair follicles behind the ear of the left ear, which will show up as single or multiple red papules and symptoms of pain behind the ear of the left ear. When patients experience pain behind the left ear, it is recommended that they go to the hospital in time for examination and diagnosis, and under the guidance of the doctor for treatment.