No Pole or Heartbeat

I’m 8wks and 2 days. Measuring at 5wks and 6days.. They found a yolk sac but no pole or heartbeat.. My HCG level is (17,626)!! I don't have any bleeding just a lil back pain which is normal for me. But I have a short cervix!!! Could that be the reason I’m measuring wrong or am I having a miscarriage!!! Doctor said not to worry but it’s not a miscarriage and that I was Having a normal pregnancy