Anxiety or Panic Disorder: This is a common cause of shortness of breath in young adults, especially without other symptoms like cough, fever, or tachycardia. The absence of these symptoms and the demographic of the patient make this a plausible first consideration.

Pulmonary Embolism: Although not immediately suggested by the lack of tachycardia, pulmonary embolism can present with shortness of breath alone and is a critical consideration in any differential for dyspnea.
Cardiac Conditions (e.g., Mitral Valve Prolapse, Cardiomyopathy): These can cause shortness of breath without other symptoms, especially if the patient is not exerting themselves at the time of evaluation.
Anemia: Can cause shortness of breath due to decreased oxygen-carrying capacity of the blood, without necessarily causing other symptoms like cough or fever.

Pneumothorax: A spontaneous pneumothorax can occur in young, healthy individuals without a history of lung disease and can present with sudden onset of shortness of breath.
Cardiac Tamponade: Although rare, it is a life-threatening condition that can present with shortness of breath and requires immediate intervention.
Hypertrophic Cardiomyopathy: A leading cause of sudden cardiac death in young athletes, which can present with exertional dyspnea.

Interstitial Lung Disease: Conditions like sarcoidosis or idiopathic pulmonary fibrosis are less common in young adults but can cause shortness of breath.
Neuromuscular Disorders (e.g., Myasthenia Gravis): Can cause respiratory muscle weakness leading to shortness of breath, though these are relatively rare.
Thyroid Disorders: Both hyperthyroidism and hypothyroidism can lead to shortness of breath, though other symptoms are typically present as well.