Costochondritis: This condition, also known as Tietze's syndrome, involves inflammation of the cartilage that connects the ribs to the breastbone. It can cause sharp pains or aching under the breast and is often exacerbated by movement or deep breathing. The pain typically resolves on its own within a few days, matching the described duration.

Musculoskeletal strain: Strain or pull in the muscles between the ribs or in the chest wall can cause pain that feels like it's under the breast. This type of pain can last for several days and is usually related to physical activity or posture.
Gastroesophageal reflux disease (GERD): GERD can cause chest pain that might be perceived as being under the breast, especially if the pain is on the left side. The pain can be intermittent and may resolve with treatment or on its own.
Breast tissue issues: Cysts, mastitis, or other breast tissue problems can cause localized pain. However, these conditions often have additional symptoms such as swelling, redness, or a palpable mass.

Myocardial infarction (MI) or acute coronary syndrome: Although less common in women and often presenting with different symptoms than in men, cardiac issues can cause pain that might be felt under the breast, especially if it's related to the heart's location under the left breast. It's crucial to consider cardiac causes, especially in patients with risk factors.
Pulmonary embolism: A blood clot in the lungs can cause sudden, severe chest pain that might radiate or be felt under the breast. This is a medical emergency.
Pneumonia or pleuritis: Inflammation of the lung or the lining around the lung can cause sharp chest pain that worsens with deep breathing or coughing, potentially felt under the breast.

Sickle cell crisis: In individuals with sickle cell disease, a crisis can cause severe pain episodes, including chest pain, due to sickling of red blood cells.
Tumors: Although rare, tumors of the breast, lung, or chest wall can cause persistent or intermittent pain under the breast.
Herpes zoster (shingles): Reactivation of the varicella-zoster virus can cause a painful rash that might be confined to a small area under the breast if it follows a dermatomal distribution.