You might possibly have some cardiac issues but not strong enough history to tell you do have problem. At the end f the day it could all be related to anxiety
. Second thing is if you are vey thinly built its natural to have feeling of heart beats on laying down.
Please have physical examination and evaluation by physician or cardiologist. If they find anything abnormal than you have to do some basic tests like ECG echocardiogram
. If everything is normal you need to build up stamina.