Musculoskeletal Strain: This is often the most common cause of SCM (sternocleidomastoid) pain and can be related to strain or overuse. The left axilla tenderness could be referred pain or a separate musculoskeletal issue.

Cervical Spondylosis: This condition can cause neck pain that might radiate to the SCM and potentially cause referred pain in other areas due to nerve compression or irritation.
Thoracic Outlet Syndrome: This condition involves compression, injury, or irritation of the nerves and/or blood vessels (arteries and veins) in the thoracic outlet area, which can cause pain in the neck, shoulder, and arm, potentially affecting the SCM and causing axillary tenderness.
Costochondritis: Inflammation of the cartilage that connects the ribs to the breastbone can cause chest pain and might refer pain to the axilla or be associated with musculoskeletal strain in the neck.

Acute Coronary Syndrome: Although less likely, chest pain or discomfort that radiates to the arm or neck could be a sign of a heart attack. The location of the pain (right SCM and left axilla) is less typical but should not rule out this diagnosis without proper evaluation.
Pulmonary Embolism: Sudden onset of chest pain and shortness of breath could indicate a pulmonary embolism. While the symptoms provided do not directly suggest this, any acute chest or arm pain warrants consideration of this potentially life-threatening condition.
Aortic Dissection: Severe, tearing chest pain that radiates to the back or arm could indicate an aortic dissection, a life-threatening condition that requires immediate medical attention.

Polymyalgia Rheumatica: An inflammatory condition that causes muscle pain and stiffness, typically in the neck, shoulder, and hip areas. It's less common but could potentially cause the symptoms described.
Lymphoma: Enlarged lymph nodes in the neck or axilla could cause localized pain and tenderness. While rare, it's an important consideration in the differential diagnosis, especially with persistent or unexplained symptoms.
Neurological Conditions: Such as multiple sclerosis or peripheral neuropathy, which could cause a wide range of symptoms including pain and tenderness in various parts of the body, including the SCM and axilla.