Maybe ask whomever prescribed the marijuana why you would have such a reaction but I suspect it is temporary. Drink lots of water. It will help you feel better.
I have really bad anxiety and I definitely can't smoke marijuana, makes anxiety much worse and causes my heart to go way too fast It's been a long time but if I remember correctly it can take pot a couple of days to get out of your system, so it could be partly that, but my experience usually is that it's anxiety. When my heart rate gets up and I get worried about it, it's going to stay elevated. Try to quit worrying about it, and don't smoke anymore.