Lymphadenitis or Lymphadenopathy: This condition, which involves the inflammation or enlargement of lymph nodes, is a common cause of lumps behind the ear. The pain behind the ear, in the jaw, and down the neck can be attributed to the inflammation of lymph nodes in the cervical chain, which can refer pain to these areas due to shared nerve roots.

Mastoiditis: An infection of the mastoid bone, located behind the ear, can cause a lump and pain in the area, radiating to the jaw and neck due to the involvement of surrounding tissues and nerves.
Parotid Gland Disorders: Conditions affecting the parotid gland, such as parotitis (inflammation of the gland) or a parotid tumor, can present with a lump behind the ear and pain that may radiate to the jaw and neck.
Temporomandibular Joint (TMJ) Disorder: While primarily affecting the jaw, TMJ disorders can cause referred pain to the ear and neck area, and in some cases, may be associated with swelling or a lump if there is significant inflammation.

Malignancy (e.g., Lymphoma, Metastatic Cancer): Although less common, a lump behind the ear could be a sign of a malignant process, such as lymphoma or metastatic cancer to the lymph nodes or parotid gland. Early detection is crucial for treatment and prognosis.
Abscess or Deep Neck Infection: An abscess or deep neck infection can present with a lump and severe pain, and if not promptly treated, can lead to serious complications, including airway obstruction and sepsis.
Cervical Spine Disease: Conditions affecting the cervical spine, such as a herniated disc or spinal stenosis, can cause pain that radiates to the neck and can be referred to the ear area, although a lump would be less common.

Branchial Cleft Cyst: A congenital anomaly that can present as a lump along the side of the neck or behind the ear, which may become infected and cause pain.
First Branchial Cleft Anomalies: Similar to branchial cleft cysts but involving the first branchial cleft, these can present with a lump and pain behind the ear or in the parotid region.
Eagle Syndrome: A condition characterized by an elongated styloid process or calcified stylohyoid ligament, which can cause pain in the ear, face, and neck, although a distinct lump behind the ear would be less typical.