What Causes Pounding Heart Beats While Doing Exercise?

I have had a stent in the main artery of my heart since 2008. I am 60 with a strong paternal history of heart disease and premature deathe. I exercise regularly, 2 hours 4-5 days a week. I experienced pounding heart beats for the first time yesterday for about 8 hours after I exercised. My heartbeat was not irregular nor elevated. I am a bit concerned because this has never happened to me before.