What Causes Resting Heart Rate To Drop?

my resting heart rate is normaly around 60 to 70.....but for the last couple of months it has been falling into the lower 50's. i suffer from anxiety which i just started taking xanax for.( 1mg daily). i was concerned for my heart rate so i talked to my doctor. he told me it was just my anxiety and not to worry about it. a week later after being very tired for a few days straight and waking with my heart rate at 47 i went to the er. they ran every test that they could.... thyroid, supplement deficiency, ekg, checked my heart enzymes, x-ray, and a couple of others. all came back normal. all they could say is that i had sinus bradycardia. my question is even though my doc says i am fine and all the test come back normal, should i see a cardiologist just to make sure there is not a underline problem with my heart? i just think it is funny how all of a sudden my resting heart rate has dropped so much. thank you!!!