Costochondritis: This condition is characterized by inflammation of the cartilage that connects the ribs to the breastbone, which can cause sharp chest pain. Given the history of a hematoma to the chest 3 weeks ago, it's plausible that the trauma led to costochondritis. The use of Coumadin for atrial fibrillation (afib) increases the risk of bruising and hematoma, which could further contribute to the development of costochondritis.

Musculoskeletal strain: The trauma from the hematoma could have also led to a musculoskeletal strain in the chest wall, causing sharp pain.
Pulmonary embolism (PE): Although less likely given the sharp nature of the pain, patients on Coumadin for afib are at increased risk of thromboembolic events, including PE. However, the presence of a hematoma and the sharp nature of the pain make this less likely.
Pericarditis: This condition, characterized by inflammation of the pericardium, can cause sharp chest pain. The use of anticoagulants like Coumadin might increase the risk of pericardial effusion, which could lead to pericarditis.

Aortic dissection: Although rare, aortic dissection is a life-threatening condition that requires immediate attention. The sharp, tearing nature of the pain (if present) and the patient's history of afib and anticoagulation therapy make it crucial not to miss this diagnosis.
Pneumothorax: Trauma to the chest can lead to pneumothorax, especially in patients on anticoagulants, which could exacerbate any bleeding into the thoracic cavity.
Myocardial infarction: While the sharp nature of the pain might not typically suggest myocardial infarction (MI), patients with afib are at an increased risk of MI. The presence of chest pain, regardless of its character, warrants consideration of MI, especially in high-risk patients.

Sternal fracture: Direct trauma to the chest could result in a sternal fracture, which would cause sharp chest pain. However, this would typically be associated with more significant trauma than what might be expected from a hematoma.
Tietze's syndrome: A rare condition characterized by inflammation of the cartilages and surrounding tissues of the ribs, similar to costochondritis but less common and often involving the upper ribs.
Thoracic spine fracture: Osteoporotic or traumatic fractures of the thoracic spine could cause sharp chest pain, especially if the fracture involves the anterior aspects of the vertebrae or the ribs.