Anemia: This is a common condition in women of childbearing age, especially if they have heavy menstrual periods. Anemia can lead to a decrease in oxygen delivery to tissues, resulting in shortness of breath during exertion. The normal blood pressure and slightly elevated pulse rate (90 bpm) could be compensatory mechanisms to increase oxygen delivery.

Asthma: Although asthma can present at any age, it is a common cause of exertional dyspnea. The symptoms can be triggered by physical activity, and an elevated pulse could be a response to the respiratory distress.
Chronic Obstructive Pulmonary Disease (COPD): Although less common in younger individuals, COPD can occur in anyone with a history of smoking or significant exposure to lung irritants. It would cause shortness of breath during exertion due to airflow limitation.
Deconditioning: A sedentary lifestyle can lead to decreased cardiovascular fitness, resulting in shortness of breath and an elevated heart rate during physical exertion.

Pulmonary Embolism (PE): Although less likely, PE is a life-threatening condition that can present with exertional dyspnea. It requires immediate diagnosis and treatment. The absence of other symptoms like chest pain or syncope does not rule out PE.
Cardiac Conditions (e.g., Hypertrophic Cardiomyopathy, Coronary Artery Disease): These conditions can lead to exertional dyspnea due to impaired cardiac output or ischemia. They are critical to diagnose early due to their potential for severe outcomes, including sudden cardiac death.
Pneumonia or Other Infectious Causes: Infections can cause shortness of breath, and while the patient's presentation might not strongly suggest an acute infection, it's crucial not to miss pneumonia or other respiratory infections that could rapidly progress.

Mitral Valve Prolapse or Other Valvular Heart Diseases: These can lead to exertional dyspnea if they significantly impair cardiac function. They are less common but should be considered, especially if other signs or symptoms suggestive of cardiac disease are present.
Interstitial Lung Disease: Conditions like sarcoidosis or idiopathic pulmonary fibrosis can cause shortness of breath during exertion. They are less common and might require specific diagnostic tests for identification.
Thyroid Disorders: Both hyperthyroidism and hypothyroidism can lead to respiratory symptoms, including shortness of breath, though they are less directly linked to exertional dyspnea compared to other diagnoses on this list.