Parotitis: This is the most likely diagnosis given the location of the swelling (below and behind the ear) and the absence of systemic symptoms such as fever. Parotitis refers to inflammation of the parotid gland, which can be caused by viral or bacterial infections, obstruction of the parotid duct, or other factors. The fact that the swelling is tender and not erythematous supports this diagnosis.

Lymphadenitis: Inflammation of the lymph nodes in the neck, which could be due to a variety of causes including infection, can present with tender swelling in the area below and behind the ear. The absence of fever and ear or mouth symptoms makes this less likely than parotitis but still a consideration.
Benign Parotid Gland Tumors: Such as pleomorphic adenoma, could present as a single, tender swelling below and behind the ear, although they are typically painless.
Sialadenitis of the Submandibular Gland: Although less common than parotitis, inflammation of the submandibular gland could potentially cause swelling in this area, especially if the swelling extends to the cheek.

Malignant Tumors: Including parotid gland tumors or metastatic disease to the lymph nodes in the neck. Although rare, these conditions are critical to diagnose early.
Tuberculous Lymphadenitis: Can present with chronic swelling and is important to consider, especially in areas with high prevalence of tuberculosis.
Sjögren's Syndrome: An autoimmune disorder that can cause parotid gland swelling among other symptoms.

Actinomycosis: A rare bacterial infection that can cause chronic swelling and abscess formation in the face and neck.
Kimura Disease: A rare, chronic inflammatory disorder that can cause lymphadenopathy and swelling in the head and neck region, often accompanied by eosinophilia.
Branchial Cleft Cyst: A congenital anomaly that can present as a swelling in the neck, typically along the anterior border of the sternocleidomastoid muscle, but can be considered in the differential for swellings in this area.