How many weeks do you think you are? This happened to me too. When I went for my first ultrasound I didn't even get to see a heartbeat. There was only a sac. My bloodwork showed my hcg levels were lower than normal that day. When I had my bloodwork done again they had gone up, but my progesterone had gone down.
They told me I would miscarry.
I ended up switching doctors (for many reasons), and my new doctor told me not to place too much stock in the numbers alone.
I am 33 weeks this week, and I have had no complications. :)
I know it is stressful. Worrying about miscarrying is the worse feeling. Good luck!