I would try and wait it out, cause who knows, maybe this is the right med for you and you just have to deal with the side effects, 3-days ispretty early to even start seeing any effects, so maybe your body is just rejecting it right off the bat?
As long as your heart rate returns to normal you don't really have anything to worry about, yea it's a bad feeling, that increases anxiety, but as long as it does come back down to you will be fine. Only when it's at a constant state of rapid beats should you get it checked out as soon as possible. You can always call your doctor though and tell them how you are feeling.