Musculoskeletal strain or sprain: Given the location over the spine and the symptoms of pain radiating to the right arm, a musculoskeletal issue such as a strain or sprain is a plausible explanation. The increasing swelling and redness could be indicative of inflammation.

Herniated disk: The pain radiating to the right arm suggests nerve involvement, which could be due to a herniated disk pressing on a nerve root.
Osteomyelitis or discitis: Infection of the bone or disc could explain the swelling, redness, and pain, especially if the patient has a history of recent trauma, injection drug use, or immunocompromised state.
Soft tissue tumor: A benign or malignant tumor of the soft tissues (e.g., lipoma, sarcoma) could present as a mass with increasing size and associated pain.

Spinal epidural abscess: Although less common, this condition is a medical emergency that requires prompt diagnosis and treatment to prevent neurological deterioration and potential paralysis.
Spinal cord compression: Any condition causing compression of the spinal cord (e.g., tumor, abscess, herniated disk) is a medical emergency that requires immediate attention to prevent permanent neurological damage.
Osteoporotic compression fracture: Especially in postmenopausal women or those with risk factors for osteoporosis, a compression fracture could present with sudden onset of back pain and should not be missed due to the potential for further fractures and morbidity.

Multiple myeloma: A malignancy of plasma cells that could present with a solitary plasmacytoma in the spine, causing pain and neurological symptoms.
Eosinophilic granuloma: Part of the spectrum of Langerhans cell histiocytosis, which can cause vertebral lesions and present with pain and neurological symptoms.
Spinal metastasis: Although more common in older adults or those with known primary malignancies, metastatic disease to the spine can present with pain, neurological symptoms, and a palpable mass.