dude that shit has hella happened to me. ME and my buddy were smoking a blunt and after that shit was done his friend comes in with another dub of white rhino and we smoked it all then afterwards i was sooooooooooooo stoned. All of a sudden i felt like i couldnt breathe and my heart was beating sooooo fast. I asked my friend who wasnt high if it was beating fast he said no it seems normal to me so i think that it happens becuase ur just so high u notice it. I think its just part of being high haha i dont even trip anymore i just say to myself its not even fast ur just stoned.