Are you sure of your dates? Did the sac measure at 10 weeks? If so, it sounds a lot like a molar pregnancy which is where the gestational sac will form and grow, but the fetus does not.
If you're 10 weeks according to your last period it could be because you ovulated later than you thought, meaning you're not as far along as you thought. At my first appointment they thought I was 12 weeks 4 days by my last period, but it turned out I was 7 weeks 4 days. They should draw blood to check your hormone levels and give you another ultrasound in a week.
it could aslo be a blighted ovum, thats when the egg implants and grows, but only to a certain extent, the placenta can even grow but no baby ever develops, that why you can still have pregnancy symptoms and they're not picke up until later in the first trimester as the placentsa keeps the pregnancy going, it can take a while for your body to recognise that no baby has developed