Fast heart beats is called tachycardia, most common causes for tachycardia are anxiety, fear, pain, physical exercise such as running,swimming and walking upstairs, exessive alcohol, smoking, caffeine intake as exess coffee or cola and choclate.
Some medical disorders as fever, anaemia, hyperthyroidism (exessive secretion of thyroid hormones in blood), panic attacks, hypotension, shock.
Heart problems may be also responsible as SVT (supraventricular tachycardia) and A.F (atrial fibrillation) and ventricular tachycardia (a life threatening condition).
Some medications also specially bronchodilators used in Asthma as inhalers and pills, and medications containing caffeine may cause tachycardia as that used in the treatment of headache and migraine.
Rare diseases as pheochromocytoma may cause attacks of palpitations together with other symptoms of sympathetic overactivity as sweating, pallor and markedly elevated blood pressure.