Recurrent or Persistent Pneumonia: The patient was recently discharged after being treated for pneumonia and now returns with similar symptoms, suggesting that the initial infection may not have been fully cleared or has recurred.

Pulmonary Embolism: Given the recent history of pneumonia and hospitalization, the patient is at increased risk for venous thromboembolism, which could manifest as shortness of breath and hypoxia.
Chronic Obstructive Pulmonary Disease (COPD) Exacerbation or Asthma: If the patient has a history of COPD or asthma, an exacerbation could present with worsening shortness of breath and decreased oxygen saturation.
Pleural Effusion: A complication of pneumonia, which could lead to shortness of breath and decreased oxygen saturation if large enough to compromise lung expansion.

Pulmonary Embolism (mentioned again due to its critical nature): Although listed under other likely diagnoses, it's crucial to emphasize the importance of not missing this potentially fatal condition.
Septic Shock or Sepsis: If the pneumonia was not fully treated or if there's a new infectious source, the patient could be developing sepsis, which is life-threatening.
Cardiac Causes (e.g., Myocardial Infarction, Cardiomyopathy): Cardiac issues can present with shortness of breath and hypoxia, and missing these diagnoses could be fatal.

Cystic Fibrosis Exacerbation: If the patient has undiagnosed cystic fibrosis, an exacerbation could present with respiratory symptoms similar to those described.
Interstitial Lung Disease: Certain conditions like sarcoidosis or idiopathic pulmonary fibrosis could present with progressive shortness of breath and hypoxia, though they would be less likely given the acute presentation.
Pneumothorax: A potential complication of pneumonia, though less common, which could lead to acute shortness of breath and hypoxia if significant.