Thoracic Outlet Syndrome (TOS): This condition is characterized by compression of the nerves and/or blood vessels that pass into the thoracic outlet (the space between your collarbone and first rib). It is a common cause of arm pain and weakness, especially after activities that involve throwing, which can exacerbate the compression. The fact that function is regained suggests a transient compression that resolves with rest.

Rotator Cuff Injury: Injuries to the rotator cuff muscles or tendons can cause pain and weakness in the arm, especially after activities that involve throwing. These injuries can lead to temporary dysfunction that improves with rest.
Ulnar Neuropathy: Compression or irritation of the ulnar nerve can cause pain, weakness, and shaking in the arm, particularly affecting the hand and forearm. Throwing activities might exacerbate the symptoms due to increased pressure on the nerve.
Overuse or Repetitive Strain Injury: Repeated throwing can lead to overuse injuries of the muscles, tendons, or ligaments in the arm, resulting in pain, weakness, and sometimes shaking. These symptoms often improve with rest and appropriate management.

Cervical Radiculopathy: Compression or irritation of the nerve roots in the cervical spine can cause arm pain, weakness, and numbness. While less likely, missing this diagnosis could lead to significant and potentially permanent neurological damage if not addressed promptly.
Vascular Injury or Thoracic Aortic Dissection: Although rare, a vascular injury or a condition like thoracic aortic dissection could present with arm pain and weakness. These conditions are medical emergencies that require immediate attention to prevent severe complications or death.

Multiple Sclerosis: An autoimmune disease that affects the brain and spinal cord, leading to a wide range of neurological symptoms, including arm weakness and shaking. The symptoms described could be part of a relapsing-remitting pattern, but this would be an uncommon initial presentation.
Amyotrophic Lateral Sclerosis (ALS): A progressive neurological disease that affects nerve cells in the brain and spinal cord, leading to loss of muscle control. ALS is rare and typically presents with more widespread and progressive symptoms, but early stages could potentially mimic the described symptoms.
Peripheral Nerve Tumors: Benign or malignant tumors affecting the nerves of the arm could cause pain, weakness, and shaking. These are rare and would typically have a more gradual onset of symptoms.