Infected Sebaceous Cyst: This is a common condition that can cause a tender lump behind the ear. The sebaceous cyst can become infected, leading to pain, redness, and swelling.

Furuncle or Carbuncle: A furuncle (boil) or carbuncle (cluster of boils) can occur behind the ear, especially in areas with high sebaceous gland concentration, causing a tender lump.
Lymphadenitis: Inflammation of the lymph nodes behind the ear can cause tender lumps, often associated with infection or inflammation in the face or scalp.
Abscess: A collection of pus in the tissue behind the ear can cause a tender lump, which may be related to a bacterial infection.

Mastoiditis: An infection of the mastoid bone, located behind the ear, can cause a tender lump and is a serious condition that requires prompt medical attention to prevent complications like meningitis or brain abscess.
Parotid Gland Tumor: Although less common, a tumor of the parotid gland, which is located near the ear, can present as a lump and must be considered to rule out malignancy.

Branchial Cleft Cyst: A congenital anomaly that can present as a lump behind the ear, typically not tender unless infected.
First Branchial Cleft Anomalies: These are rare congenital anomalies that can present with a lump or discharge behind the ear.
Metastatic Disease: In rare cases, a lump behind the ear could be a metastasis from a distant primary cancer, highlighting the importance of a thorough medical history and examination.