Cyst or abscess: This is a common cause of localized swelling, especially if there's a history of trauma, infection, or a pre-existing cystic structure like a sebaceous cyst. The location below the occipital region is prone to such formations due to the presence of hair follicles and sebaceous glands.

Traumatic hematoma: A collection of blood outside of blood vessels, often caused by injury, which could lead to swelling in the affected area.
Furuncle or carbuncle: An infection of the hair follicle or a group of hair follicles, which can cause localized swelling, redness, and pain.
Lipoma: A benign tumor composed of fat tissue, which can grow under the skin and cause swelling.

Meningioma: Although rare, a meningioma (a tumor that forms in the meninges) could potentially cause swelling if it grows large enough to exert pressure on the scalp or if it has a component that extends through the skull.
Subgaleal hemorrhage: A serious condition where blood collects between the skull and the scalp, potentially life-threatening and requiring immediate medical attention.
Osteomyelitis of the skull: An infection of the bone, which could lead to swelling, pain, and other systemic symptoms.

Dermoid cyst: A rare type of cyst that can form on the scalp, containing skin and sometimes hair, which can become infected or inflamed and cause swelling.
Epidural or subdural hematoma: Although more commonly associated with head trauma and symptoms like headache and altered mental status, in rare cases, they might present with localized swelling if there's a significant mass effect.
Neurofibroma: A benign nerve sheath tumor that can cause swelling, especially in the context of neurofibromatosis type 1.