Empty gestational sac - advice??

I had my first ultrasound today. They think I'm 5w3d based on what they saw. There was an  empty gestational sack. I'm freaking out because I had a positive OPK last month that would have me at 7w1d. Based on my LMP id even be at 8 weeks.
 I just had blood work done and will have to get it done again in a couple days to measure change in hcg. I feel hopeless.
 Has anyone had something similar happen to them and it turned out okay?? I really need some advice or comforting. I'm a wreck.. Thanks in advance!