Musculoskeletal strain or injury: This is often the most common cause of sudden onset severe arm and shoulder pain, especially if there was a recent history of physical activity or trauma. The pain can be acute and severe due to muscle or tendon injury.

Rotator cuff injury: Injuries to the rotator cuff, which includes the muscles and tendons surrounding the shoulder joint, can cause severe pain in the shoulder and arm. This is common in individuals who perform repetitive motions or have experienced trauma to the shoulder.
Bursitis or tendinitis: Inflammation of the bursae (fluid-filled sacs that cushion bones, tendons, and muscles near joints) or tendons can cause significant pain and stiffness in the shoulder and arm.
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 in your shoulders and arms, especially with certain movements.

Myocardial infarction (heart attack): Although less common, myocardial infarction can present with arm pain, especially if the pain is radiating from the chest. It's crucial to consider cardiac causes, especially in patients with risk factors for coronary artery disease.
Pulmonary embolism: While typically presenting with respiratory symptoms, a pulmonary embolism can occasionally cause referred pain to the arm or shoulder, particularly if the embolism affects blood flow in a way that irritates nerve roots.
Stroke or transient ischemic attack (TIA): In rare cases, a stroke or TIA can present with sudden onset of severe arm pain, especially if the pain is accompanied by neurological deficits such as weakness or numbness.

Pancoast tumor: A type of lung cancer located at the top of the lung that can cause shoulder and arm pain due to invasion of the brachial plexus (nerves that run from the spinal cord to the arm).
Cervical radiculopathy: Compression or irritation of the nerves exiting the cervical spine can cause pain, numbness, and weakness in the arm. While not as common as other musculoskeletal causes, it's an important consideration, especially with neck movements exacerbating symptoms.
Complex regional pain syndrome (CRPS): A chronic pain condition characterized by severe, burning pain at the site of an injury, which can sometimes be triggered by a minor injury. It's rare but should be considered in cases of disproportionate or prolonged pain.