I'm not sure what your doctor is thinking... A baby's heart has JUST started beating at 5 weeks (sometimes later) and they start slow and gradually speed up. My baby's heart rate was 103 at 6 weeks and my doctor said that was fine. It went on to increase to 150 at 7.5 weeks and then 180 at 8.5 weeks. Since then, his heart rate has remained around 150-160 (I'm 23 weeks). Enjoy your pregnancy and don't stress...hearing the heart beat is often a very good sign. =)
http://www.i-am-pregnant.com/encyclopedia/Pregnancy/Fetal-Heart-Rate This link should answer your question :) I think you are right on track since you are still in very early pregnancy.