Muscle strain: This is the most likely diagnosis due to the common occurrence of muscle overuse or strain in the upper arm, which can cause pain and a sense of pressure without any history of trauma.

Tendinitis: Inflammation of the tendons in the upper arm, such as the biceps or triceps tendons, can cause pain and pressure.
Repetitive strain injury: Repetitive activities or movements can cause strain on the muscles and tendons of the upper arm, leading to pain and pressure.
Thoracic outlet syndrome: Compression of the nerves or blood vessels in the thoracic outlet can cause pain and pressure in the upper arm.
Cervical radiculopathy: Nerve root compression in the cervical spine can cause referred pain and pressure in the upper arm.

Acute coronary syndrome: Although less common, upper arm pain can be a referred symptom of a heart attack, especially if the pain is accompanied by other symptoms such as chest pain, shortness of breath, or dizziness.
Pulmonary embolism: A blood clot in the lungs can cause referred pain in the upper arm, especially if the clot is large and causes significant strain on the lungs.
Aortic dissection: A tear in the aorta can cause severe pain and pressure in the upper arm, as well as other symptoms such as chest pain and shortness of breath.

Pancoast tumor: A rare type of lung cancer that can cause pain and pressure in the upper arm due to compression of the nerves and blood vessels.
Brachial plexus neuropathy: Inflammation or damage to the brachial plexus nerves can cause pain and pressure in the upper arm.
Osteoid osteoma: A rare type of bone tumor that can cause pain and pressure in the upper arm, especially at night.
Ehlers-Danlos syndrome: A rare genetic disorder that can cause joint hypermobility and skin elasticity, leading to pain and pressure in the upper arm due to joint instability.