I thought the same thing, but I ddin't think you would be able to see a heartbeat that early. I could clearly see the flicker, but she used a pencil to show me how big the sac should be and it was no where near what she was pointing at. I also have a retroverted uterus so hopefully this is playing a role also. I go back in two weeks for another scan.
Perhaps you are not as far along as you thought...maybe you ovulated later in your last cycle and therefore you are 4-5 weeks rather than the 6 weeks you thought. Not much you can do other than try to stay positive and wait for your next ultrasound which I would assume would be in a week or 2 to follow up on the situation/your doctor's concern.