The causes of a lump behind the ear may include sebaceous gland obstruction, folliculitis, enlarged lymph nodes and so on. 1. Sebaceous gland obstruction: commonly found in adolescents with high endocrine secretion, sebaceous gland excretory tube blockage, local sebum retention caused by sebaceous gland cysts behind the ear. This leads to the appearance of symptoms of a pimple behind the ear. 2. Folliculitis: caused by bacterial attack on hair follicles. It tends to occur in the area behind the ear, leading to the appearance of bumps behind the ear. At the same time, it can make the skin appear red, swollen, painful symptoms, and in severe cases, it can even lead to pus, and only after the pus is discharged, it can be cured. Boils. 3. swollen lymph nodes: for example, when suffering from acute otitis media, mumps, pulpitis, tonsillitis and other diseases, can cause inflammation of the lymph nodes, resulting in reactive hyperplasia, lymph node enlargement phenomenon. When there is a lump behind the ear, it is recommended to go to the hospital for a clear diagnosis and treatment.