You should get an EKG to rule out any serious heart arrhythmias. I don't know the significance nor identify any etiology associated with lying down and arrhythymia although that is oftened an accurrence.
However, the following conditions should be ruled out and that includes valvular diseases such as aortic insufficiency or stenosis, atrial or ventricular septal defect, congestive heart failure, cardiomyopathy, and congenital heart disease.
Thanks for sharing, and if you have any followup questions or comments you are welcome to respond. Take care,
Ken