What Causes Heart Pounding While Lying Down?

Hi - When I lie down on my back or left side (but not on my right side) after approx. 20 minutes my heart starts to pound and my chest feels uncomfortable. My breathing is not affected at all. This has been occurring for the past 3 years or so, so I know it s not urgent, but lately these symptoms seem - stronger. Of course I can sleep on my right side, but I am just wondering why this is occurring. I am in good health - run 10 to 15 miles per week. I am 61. Any thoughts? Meg