Pericardial Effusion/Tamponade: This condition is a well-known complication post-cardiac surgery, including CABG. The symptoms of chest pain, painful breathing, and low blood pressure are classic for tamponade, which can occur due to bleeding into the pericardial space. The timing, 2 days post-CABG, also fits this diagnosis.

Pulmonary Embolism: Although less common than pericardial effusion in this context, pulmonary embolism can cause chest pain, dyspnea, and hypotension. Immobility and the post-surgical state increase the risk of venous thromboembolism.
Myocardial Infarction: Early graft occlusion or native coronary artery occlusion can lead to myocardial infarction post-CABG, presenting with chest pain and potentially hypotension if there is significant myocardial involvement.
Pneumothorax: This can occur due to barotrauma from mechanical ventilation or as a complication of the surgical procedure itself, leading to chest pain, difficulty breathing, and potentially hypotension if it is a tension pneumothorax.

Aortic Dissection: Although less common, an aortic dissection can be a catastrophic complication, especially in the context of recent cardiac surgery. It presents with severe chest pain and can lead to hypotension due to involvement of the aortic root or dissection into vital organs.
Bleeding into the Thoracic Cavity: Significant bleeding into the thoracic cavity can lead to hypovolemic shock and can compress the heart and lungs, impairing their function. This is a medical emergency requiring immediate intervention.

Pulmonary Artery Thrombosis: While less common than pulmonary embolism, thrombosis of a pulmonary artery can occur, especially in the setting of recent surgery and immobility.
Sternal Wound Infection with Mediastinitis: Early onset of sternal wound infection can lead to mediastinitis, which, although rare, can cause severe chest pain, difficulty breathing, and systemic signs of infection, including hypotension.