Hello, I think that your dates could just be off. You may not be as far along as the doctor say that you are. I would not worry, just watch it and wait. Best wishes to you! :)
If you had not had an IUI, I might be inclined to say that your dates were off.
A small sac and fetal pole is a poor prognostic sign, but a normal heart beat is a good sign. Unfortunately in your case, you are just going to have to watch and wait. If the fetus continues to grow, you should soon see a healthy baby. If it does not, it probably signals that there was a major problem with the development of the pregnancy and sometimes in these cases a miscarriage is a blessing in disguise.
I wish you the best!