If you are feeling the heart rate, it is called palpitation. Most common cause for your age could be Hyperthyroidism. Otherwise, if some stress element is there, it could be due to Anxiety. If there are accompanying symptoms like loss of weight in spit of good appetite ot sweating on palms of hands, tremors, goitre, insomnia etc, you may get T3,T4 & TSH done along with CBC done & go to a Physician

Palpitations need detailed evaluation to rule out metabolic and cardiac causes...meet MD physician...if normal then anxiety need to be considred