You should consult a cardiologist
who may order a few tests like ECG, 2-D Echo, Holter monitoring, Treadmill test and electrophsiological studies(EP studies). The possible causes of low pulse rate in your case may be your activity level (increased vagal tone in athletes/sportspersons),poorly functioning thyroid gland(hypothyroidism
),drug induced , depressed mood or some cardiac defect(conducting system defect).Your cardiologist would be of great help to you.
Dr RAVINDRA MATHUR