Anxiety or Panic Attack: This is a common condition in young females, often presenting with symptoms of a racing heart, palpitations, and shortness of breath, triggered by stress or anxiety.

Hyperthyroidism: An overactive thyroid gland can cause an increased heart rate, among other symptoms like weight loss, heat intolerance, and tremors.
Dehydration or Electrolyte Imbalance: Dehydration or an imbalance of essential electrolytes like potassium, sodium, or magnesium can lead to palpitations and a racing heart.
Caffeine or Substance Use: Excessive consumption of caffeine, nicotine, or other stimulants can cause heart palpitations and a rapid heart rate.
Premenstrual Syndrome (PMS): Some women experience heart palpitations as part of their premenstrual symptoms, possibly due to hormonal changes.

Cardiac Arrhythmias (e.g., Supraventricular Tachycardia, SVT): Although less common, these conditions can be life-threatening if not promptly treated. They may present with sudden onset of a rapid heart rate.
Myocarditis or Pericarditis: Inflammation of the heart muscle or the sac surrounding the heart can cause a rapid heart rate and may be associated with chest pain and shortness of breath.
Pulmonary Embolism: A blood clot in the lungs can cause sudden onset of a rapid heart rate, along with shortness of breath and chest pain.
Hypertrophic Cardiomyopathy: A condition where the heart muscle becomes thickened, which can lead to arrhythmias and sudden cardiac death, especially in young athletes.

Pheochromocytoma: A rare tumor of the adrenal gland that can cause episodes of a rapid heart rate, high blood pressure, and sweating.
Thyroid Storm: A life-threatening complication of untreated or undertreated hyperthyroidism, presenting with severe symptoms including a very rapid heart rate.
Arrhythmogenic Right Ventricular Cardiomyopathy (ARVC): A rare heart condition where the muscle in the right ventricle is replaced by fat and scar tissue, potentially leading to arrhythmias and sudden death.