Happened to me during my first time smoking, and it was a panic attack. For awhile as a beginner it still happened, but I learned to live with it. Now it rarely does. if you keep your mind off it (i know, easier said than done), it will go away. There is no reason to worry about it, guys. You're not going to die. Just tell yourself that nobody has ever died from an overdose of cannabis, so why would you? Also, tell yourself that you did it to have fun, not to have a panic attack and have a shitty, tormenting time. It could be that you were overwhelmed... that's why it happened. Perhaps you've never been that high before? I think the rapid heart beat is either completely fake (as in, it's all in your head), or you're just scaring yourself, so your heart beats faster. Afterall, when you're scared, your heart does beat faster. If you're not scared, why would it? Also, Reaper666, I agree that what you said can happen, but I believe this is just the result of an overwhelming high. The trick is to calm yourself down. Cheerz.

P.S. I noticed that with sativa strains this happens more often. Try an indica strain if you just can't get this to stop.