Pulmonary Embolism (PE): Given the patient's history of total knee replacement, which is a significant risk factor for deep vein thrombosis (DVT) and subsequently PE, this diagnosis is highly plausible. The symptoms of chest pain and shortness of breath further support this possibility.

Myocardial Infarction (MI): Although the history of knee replacement points towards a thromboembolic event, MI is a common cause of chest pain and shortness of breath and should be considered, especially if the patient has risk factors for coronary artery disease.
Pneumonia: Infection can cause both chest pain and shortness of breath. Given the patient's recent surgery, hospital-acquired pneumonia is a possibility.
Chronic Obstructive Pulmonary Disease (COPD) Exacerbation: If the patient has a history of COPD, an exacerbation could present with these symptoms.

Aortic Dissection: Although less common, aortic dissection is a life-threatening condition that presents with chest pain and can cause shortness of breath. It requires immediate diagnosis and treatment.
Pneumothorax: Spontaneous pneumothorax can cause sudden onset of chest pain and shortness of breath. It's less likely but critical to diagnose promptly.
Cardiac Tamponade: This condition, characterized by fluid accumulation in the pericardial sac, can lead to cardiac compression and present with chest pain and shortness of breath. It's a medical emergency.

Pulmonary Arteriovenous Malformation: A rare condition that could potentially cause these symptoms, especially if there's a history of such malformations or if the patient has hereditary hemorrhagic telangiectasia.
Lymphangitic Carcinomatosis: Metastatic cancer to the lungs can cause shortness of breath and chest pain, but it's less common and usually associated with a known history of cancer.
Acute Coronary Syndrome due to Vasculitis: Conditions like Kawasaki disease or Takayasu arteritis can cause coronary artery inflammation leading to ACS, but these are rare and typically associated with other systemic symptoms.