Costochondritis: This is a common condition characterized by inflammation of the cartilage that connects the ribs to the breastbone, often causing chest pain that can be localized to one side. The chronic nature of the pain (3 months) and its location below the breast make costochondritis a plausible explanation.

Musculoskeletal strain: Prolonged or repetitive strain on the muscles and joints in the chest area can lead to pain. Given the duration and location of the pain, a musculoskeletal issue is a reasonable consideration.
Gastroesophageal reflux disease (GERD): GERD can cause chest pain, particularly after eating or at night, and can be localized to one side of the chest. The pain's persistence over 3 months could suggest a chronic condition like GERD.
Breast pathology: Although less common in men, breast conditions such as cysts, mastitis, or even breast cancer could cause localized pain. In women, these conditions are more likely and should be considered, especially if there are other symptoms like a lump or nipple discharge.

Myocardial infarction (MI) or acute coronary syndrome: Although the pain has been present for 3 months without improvement, cardiac causes of chest pain can sometimes present atypically, especially in women or diabetic patients. It's crucial to rule out cardiac issues due to their high morbidity and mortality.
Pulmonary embolism (PE): A PE can cause sudden onset of chest pain, but in some cases, the pain can be more chronic. Given the potential for severe consequences, PE should be considered, especially if there are risk factors such as recent travel, immobilization, or family history of clotting disorders.
Pneumonia or lung abscess: Infections in the lung can cause localized chest pain. While these conditions often present with additional symptoms like fever or cough, it's essential to consider them due to their potential severity.

Tietze's syndrome: Similar to costochondritis but involves the inflammation of the cartilages and surrounding tissues of the upper ribs. It's less common and could be considered if costochondritis is ruled out.
Sickle cell crisis: In patients with sickle cell disease, a crisis can cause severe chest pain due to vaso-occlusion in the lungs. This would be an important consideration in patients with a known history of sickle cell disease.
Thoracic outlet syndrome: This condition involves compression of the nerves and/or blood vessels that pass into the thoracic outlet (the space between your collarbone and first rib). It can cause chest pain among other symptoms and is more common in certain occupations or activities.