Not rubbing on your heart, necessarily at all. The Vagus nerve is the most common source of superimposition of an ectopic beat upon the heart's normal sinus rhythm. A number of things can cause stimulation of the Vagus nerve and, consequently, it's effect on the heart in the way of benign palpitations.

I will submit to you here that I don't believe the tension in your arm is necessarily connected with the onset of palpitations, but rather your position when seated. Remember that the GI tract is innervated by the Vagus nerve as well and when seated, air can become trapped in various ways, causing stimulation to occur at the nerve, it's signal traveling outward where it can stimuate the heart, lungs, larnyx, etc., generally resulting in palpitation events and or other sensory disturbance.

In any event, it's a benign phenomenon and will never cause you any harm whatsoever. I fully realize how frightening the events can be at times, but they are nevertheless harmless. You'll be just fine.

Best regards and Good Health