Costochondritis: This is a common condition characterized by inflammation of the cartilage that connects the ribs to the breastbone, which can cause pain in the axillary region that radiates to the breast. The pain is often sharp and worsens with movement or deep breathing.

Musculoskeletal strain: Strain or injury to the muscles or tendons in the axillary region can cause pain that radiates to the breast. This could be due to overuse, direct trauma, or poor posture.
Breast cysts or fibroadenomas: Benign breast conditions such as cysts or fibroadenomas can cause localized pain that may radiate to the axilla, especially if they become large or inflamed.
Axillary lymphadenopathy: Enlarged lymph nodes in the axilla due to infection, inflammation, or other causes can cause pain that may radiate to the breast.

Breast cancer: Although less common, breast cancer can present with pain, especially if the tumor is located in the outer part of the breast near the axilla. It's crucial to not miss this diagnosis due to its significant implications.
Acute coronary syndrome: In women, especially those with risk factors, chest pain or discomfort that radiates to the arm or breast can be a symptom of a heart attack. This is a medical emergency.
Pulmonary embolism: Severe, sudden-onset chest pain that radiates to the arm or breast, accompanied by shortness of breath, could indicate a pulmonary embolism, which requires immediate medical attention.

Tietze's syndrome: A rare condition characterized by inflammation of the cartilages of the ribs, which can cause severe pain in the chest and axillary region.
Neurogenic thoracic outlet syndrome: Compression of the nerves that pass through the thoracic outlet can cause pain in the axilla and breast area, along with other symptoms like numbness or tingling in the arm.
Sickle cell disease crisis: In individuals with sickle cell disease, a crisis can cause severe pain episodes, including pain in the chest and axillary region due to vaso-occlusive crises affecting the bones and joints.