I wouldn't think it has anything to do with the weed. As you know the weed calms you down. Not an upper that should be getting you going. Whats probably going on is a little bit of paranoia
. Your smoking
, and then think your heart is beating fast, then you start to worry that something is wrong, then that triggers a little anxiety which does make your heart beat faster.
What I would say is stop worrying, your smoking the weed to calm down, so do just that, CLAM THE F DOWN! I had a friend that got REALLY high once and had acid brownies and didn't know it. She had a BAD trip and then anytime after that she smoked weed she would freak out and have anxiety problems. So she quit. So either calm down or quit.
I used to feel kinda cold
when I would smoke and that would kinda freak me out and I would get my heart going. It happened a few times then I got over it.
Really If pot gave you rapid heart beat I would know. I have tachycardia
, had it sense I was a little kid. I have not had one episode EVER from smoking pot! Sugar and caffeine totally would do it! So make sure you don't over due the caffeine soda's when you get the munchies.
Good luck.. And just make sure not to make pot run your life and define you as a person. Besides that have fun safely.