Musculoskeletal Injury or Strain: This is often the most common cause of unilateral arm swelling and pain, especially if there's a history of recent trauma, overuse, or strenuous activity. The pain and swelling can be localized to the area of injury.

Venous Thrombosis (Deep Vein Thrombosis, DVT): Swelling and pain in one arm can be symptoms of a DVT, especially if there's a history of prolonged immobilization, cancer, or clotting disorders.
Lymphedema: This condition, characterized by the abnormal collection of protein-rich fluid in the interstitial tissue, can cause swelling. It might be secondary to surgery, radiation, or infection affecting the lymph nodes.
Rotator Cuff Tendinitis or Bursitis: Inflammation of the tendons or bursae around the shoulder joint can cause pain and swelling in the upper arm and shoulder.
Cervical Radiculopathy: Nerve root compression in the cervical spine can cause pain, numbness, and sometimes swelling in the arm, depending on the nerve root affected.

Axillary Vein Thrombosis (Paget-Schroetter Syndrome): A rare condition but critical to diagnose, as it involves thrombosis of the axillary or subclavian vein, often associated with repetitive arm movements.
Thoracic Outlet Syndrome: Compression of the nerves and/or blood vessels that pass into the thoracic outlet (the space between your collarbone and first rib) can cause pain, swelling, and other symptoms in the arm.
Infection (Cellulitis or Abscess): Bacterial infections of the skin and soft tissues can cause swelling, redness, and pain. It's crucial to identify and treat promptly to avoid complications.
Neoplasm (Tumor): Although rare, tumors affecting the lymph nodes, soft tissues, or bones of the arm can cause swelling and pain.

Complex Regional Pain Syndrome (CRPS): A chronic pain condition characterized by severe, burning pain at the site of an injury, which can also cause swelling and changes in skin temperature and color.
Sarcoidosis: A condition that can cause inflammation in various parts of the body, including the lymph nodes, which might lead to swelling in the arm if the lymph nodes in the armpit are affected.
Vasculitis: Inflammation of the blood vessels can cause a variety of symptoms, including pain and swelling in the affected limb, though it's a less common cause of unilateral arm symptoms.