Dehydration: This is a common cause of low blood pressure (hypotension) and dizziness, especially when changing positions (orthostatic hypotension). The patient's symptoms of dizziness when sitting, standing, or walking suggest a possible volume depletion or dehydration, which could be due to inadequate fluid intake or other causes like gastrointestinal losses.

Orthostatic Hypotension: This condition is characterized by a significant drop in blood pressure when standing up, leading to dizziness or fainting. It can be primary (without any underlying cause) or secondary to other conditions such as diabetes, Parkinson's disease, or certain medications, though the patient is not on any medications.
Anemia: Low hemoglobin levels can lead to decreased blood volume and oxygen delivery, causing dizziness and hypotension. This could be due to various reasons such as menstrual blood loss, gastrointestinal bleeding, or nutritional deficiencies.
Autonomic Dysfunction: Disorders affecting the autonomic nervous system, such as postural orthostatic tachycardia syndrome (POTS), can lead to orthostatic intolerance, characterized by dizziness, palpitations, and hypotension upon standing.

Cardiac Conditions: Although less likely given the patient's age and presentation, cardiac causes such as heart failure, significant valvular disease, or cardiac tamponade can lead to hypotension and dizziness. These conditions are critical and require immediate attention.
Adrenal Insufficiency: This is a life-threatening condition where the adrenal glands do not produce enough cortisol and aldosterone, leading to hypotension, dizziness, and potentially shock. It's crucial to consider this diagnosis, especially if the patient has a history of steroid use or autoimmune disorders.
Neurological Disorders: Certain neurological conditions, such as multiple system atrophy or pure autonomic failure, can cause orthostatic hypotension. While rare, these diagnoses are important not to miss due to their significant impact on the patient's quality of life and the need for specific management strategies.

Pheochromocytoma: A rare tumor of the adrenal gland that can cause episodic hypertension but also orthostatic hypotension due to volume depletion and impaired autonomic reflexes.
Hypothyroidism: Severe, untreated hypothyroidism can lead to decreased cardiac output and hypotension, though this is less common and typically associated with other systemic symptoms.
Vasovagal Syncope: While more commonly associated with transient loss of consciousness, some patients may experience dizziness and hypotension without frank syncope, especially if the episode is aborted or the patient sits or lies down promptly.