Thanks for writing to us.
Feeling of heart beats when you lie down on your stomach is common and may be totally harmless. Since heart comes more close to front chest wall in this state, you feel it contracting much more. When you suddenly wake from your early sleep this may happen at that time too.
It may be due to heightened state of anxiety or may be due to certain drugs, tea coffee etc.
Do you feel abnormal heart racing at times of exercise or using stairs? or you ever feel dizziness
, blackout, loss of consciousness
, chest discomfort
or undue shortness of breath
on exercise. if the answer is no, then probably you need not worry much. since your doctor has examined you , i guess your examination is normal. Still if you are too worried you can get an ECHO done for structural heart disease
(after discussing with your cardiologist).
Feel free to discuss more.