Rapid flutter feeling (like heartbeat )in back flank area on right side (backside). it is more rapid and strong than the heartbeat. comes and goes. ?

Muscle: this appears to be an irritable muscle causing twitching. If it is a pulsation from a large artery like the aorta, the sensations would be synchronous with your pulse which you can check at your wrist,neck or in front of your ear. I suspect that this will eventually resolve on its own. See your physician if you have any further concerns.