A small pimple behind the ear is usually caused by the following reasons: 1. Inflammatory lymph node enlargement behind the ear, which is mainly caused by upper respiratory tract infection, oral ulcers, gingivitis, skin breakage of the auricle, otitis media, etc. Bacteria invade the lymph nodes behind the ear and breed in them to form inflammatory lymph node enlargement; 2. Malignant metastatic lymph node enlargement behind the ear, which is mostly Malignant tumors of the head and face, including nasopharyngeal, tongue, and gum cancers, invade the lymph nodes behind the ear along the lymphatic vessels, resulting in cancerous enlargement; 3. Sebaceous cysts, lipomas, and fibroids behind the ear are benign tumors.