Costochondritis: This condition, also known as Tietze's syndrome, is characterized by inflammation of the cartilage that connects the ribs to the breastbone (sternum). It's a common cause of chest pain in adolescents and can cause pain under the breast.

Musculoskeletal strain: The pain could be due to a strain or pull in the muscles or ligaments under the breast, possibly from overuse or injury.
Gastrointestinal issues: Conditions like gastroesophageal reflux disease (GERD) or constipation can cause referred pain under the breast.
Breast bud tenderness: In early puberty, breast development can cause tenderness or pain, which might be perceived as being under the breast.

Pulmonary embolism: Although rare in adolescents, a pulmonary embolism can cause sudden, severe chest pain and is a life-threatening condition that must not be missed.
Pneumonia or pleurisy: Infections like pneumonia or inflammation of the lining around the lungs (pleurisy) can cause chest pain that might be felt under the breast.
Cardiac issues: Though uncommon, cardiac conditions such as pericarditis or myocarditis can cause chest pain and are critical to diagnose promptly.

Tumors: Both benign and malignant tumors can cause pain under the breast, though they are rare in adolescents.
Sickle cell crisis: In individuals with sickle cell disease, a crisis can cause severe pain episodes, including chest pain.
Inflammatory bowel disease: Conditions like Crohn's disease or ulcerative colitis can have extraintestinal manifestations, including chest pain, though this is less common.