Asthma: This is a common condition that can cause shortness of breath during exercise, which improves with rest. The fact that symptoms decrease on rest supports this diagnosis, as asthma symptoms often worsen with exercise and improve with cessation of activity and use of appropriate medication.

Chronic Obstructive Pulmonary Disease (COPD): Although less common in younger individuals, COPD can cause shortness of breath that worsens with exertion and improves with rest. A history of smoking or exposure to lung irritants would increase the likelihood of this diagnosis.
Anxiety or Panic Disorder: These conditions can manifest with shortness of breath, especially in situations that provoke anxiety, such as physical exertion. The symptoms can improve with rest and relaxation.
Deconditioning: Lack of physical fitness can lead to shortness of breath with minimal exertion, which improves as the individual rests.

Pulmonary Embolism: Although less likely, pulmonary embolism is a life-threatening condition that can cause sudden onset of shortness of breath, which may worsen with exertion. It's crucial to consider this diagnosis, especially if there are other risk factors such as recent surgery, immobilization, or family history of clotting disorders.
Cardiac Conditions (e.g., Coronary Artery Disease, Cardiomyopathy): Heart diseases can cause exertional dyspnea due to inadequate cardiac output during increased demand. These conditions are critical to identify due to their potential for severe outcomes, including heart attack or sudden death.

Vocal Cord Dysfunction: This condition can cause shortness of breath and wheezing that mimics asthma but is due to abnormal vocal cord movement. It's more common in young women and can be triggered by exercise or emotional stress.
Pulmonary Vasculitis: Inflammatory diseases affecting the blood vessels of the lungs can cause shortness of breath. These conditions are rare and often associated with systemic symptoms such as fever, weight loss, and joint pains.