Heart palpatations and being aware of one's heart beat is quite common during stress and anxiety in my experience with it. Have you discussed this with your doctor?
Sounds like it could be you anxiety about being on the new med but I would definately check with your doctor anyway, it cant' hurt. I'm sure you will be okay either way :-) .. hang in there , i'm giong through the same thing...
you may need to speak with your doctor about this side-effect ok :)