Viral Pericarditis: This condition is often seen in young adults and can present with sharp chest pain that may improve with sitting up and leaning forward, along with shortness of breath due to the pericardial effusion. The recent onset and the nature of the pain, coupled with the patient's age, make this a plausible diagnosis.

Pneumonia: Community-acquired pneumonia can present with chest pain and shortness of breath. The pain can be pleuritic, worsening with deep breathing or coughing, which is common in younger adults.
Pulmonary Embolism (PE): Although less common in young adults without risk factors, PE should be considered, especially if there's a history of recent travel, immobilization, or family history of clotting disorders. The presentation can be with sudden onset of chest pain and shortness of breath.
Asthma or COPD Exacerbation: These conditions can cause chest tightness and shortness of breath, especially if the patient has a history of respiratory disease. However, the chest pain would more likely be described as tightness rather than pain.

Myocardial Infarction (MI): Although rare in young adults, MI can occur, especially with risk factors such as drug use (e.g., cocaine), hypercoagulable states, or premature coronary artery disease. It's crucial to rule out MI due to its high mortality if untreated.
Aortic Dissection: This is a medical emergency that presents with severe, tearing chest pain radiating to the back. It's rare in young adults without predisposing conditions like Marfan syndrome or hypertension but is critical not to miss.
Pneumothorax: Spontaneous pneumothorax can occur in tall, thin males and presents with sudden onset of chest pain and shortness of breath. It's a medical emergency requiring prompt intervention.

Sarcoidosis: This autoimmune disease can affect the lungs and heart, leading to chest pain and shortness of breath, but it's less common and typically presents with other systemic symptoms.
Lymphoma: Although rare, lymphoma can cause mediastinal masses leading to chest pain and shortness of breath due to compression of airways or major vessels.
Tuberculosis: Reactivation or primary TB can present with chest pain and shortness of breath, especially in endemic areas or in individuals with compromised immune systems.