Doesn't add up and I'm scared.

Elicia • Mother to a few angels above. 👼🏻 And 1 rainbow baby girl. 🌈
     I went in for an ultrasound today. I should be 10 weeks and 2 days.
     The ultrasound only showed a sac measuring at 5 weeks. 
     It doesn't make any sense. I had my first positive hpt exactly 3 weeks after my last period (October 19) and a positive blood test at 4 weeks. Which was 7 weeks ago and 6 weeks ago. 
     They took blood and I won't get the results back until next Tuesday.
     I'm scared. I already had a mmc a few months ago..