I can think of few common
causes related to the
heart, so if you are pursuing the idea that it is
heart related, I would not rule out an uncommon cause.
As a mere arrhythmia patient, I am ill-equipped to deal with the diagnosis of something out of the ordinary (although there are other among us here whose ideas have really shined in the past). As for a potential common arrhythmia issue, the only thing that comes to mind is some type of paroxysm of tachycardia.