If your lazy and/or fat your heart has to beat faster than it would if you were healthy. So even at rest, if your just sitting there smoking, and you don't generally move much anyway, your probaly going to suffer. Try getting certain strains of weed and figuring out what is making your heart race so much, besides you and your thoughts helping you to think that you are going to die any minute, its always good to figure what is a good strain for you.... and there are alot of strains about that don't induce that kind of undesirable effect.