Could be anxiety or too much caffeine. On a more serious note, there are heart conditions that cause abnormally fast heart rate (tachycardia). Any heart rate over 100 is considered tachycardia, your heart rate gets over 100 easily when your active, it should be anywhere between 60-99 when your resting. If your heart rate is over 150, this is called supra-ventricular tachycardia (SVT) when its this fast it is important to go to the ER as your heart may not be able to effectively pump blood at this rate, and may become life threatening, wolf parkinson white syndrome can cause SVT and usually requires surgical intervention. If you experience a fast heart rate and/or palpitations often you should get this checked out, especially if it is accompanied by chest pain/difficulty breathing.