Musculoskeletal strain: This is the most common cause of muscle spasm and tightness in the upper back, often due to overuse, poor posture, or direct injury to the muscles between the shoulder blades.

Thoracic outlet syndrome: Compression of the nerves and/or blood vessels between the collarbone and first rib can cause pain and muscle spasm in the upper back and shoulder area.
Scapulothoracic bursitis: Inflammation of the bursae between the scapula and thoracic wall can lead to pain and stiffness in the area.
Myofascial pain syndrome: This condition involves pain and inflammation of the muscles and their surrounding connective tissue, which can cause localized pain and muscle spasm.

Pulmonary embolism: Although less common, a pulmonary embolism can cause sudden onset of back pain, and it is crucial to rule out this potentially life-threatening condition.
Pneumonia or pleurisy: Infections like pneumonia or inflammation of the pleura can cause sharp pain in the upper back that worsens with deep breathing.
Spinal fracture or osteoporosis: Especially in older adults or those with a history of osteoporosis, a fracture in the thoracic spine could cause sudden onset of back pain.

Tietze's syndrome: A rare condition characterized by inflammation of the cartilages and surrounding tissues of the ribs, which can cause chest and upper back pain.
Sickle cell crisis: In individuals with sickle cell disease, a crisis can cause severe pain episodes, including in the upper back.
Pancoast tumor: A rare type of lung cancer located at the top of the lung, which can cause shoulder and upper back pain due to local invasion of structures.