That happened to me too. I was supposed to be fourteen weeks but only measured nine. It was a missed miscarriage. Was there a heartbeat?
My first pregnancy a few months ago, I was suppose to be 10 weeks. But I only measured 5. I had a "missed miscarriage", and a surgery to remove it called d&c. Being off by that many weeks isnt a good sign. Im sorry and I hope her dates are just wrong.
If she was 10 weeks by her last month period, then it's not a big deal. I was 12 weeks by my lmp and after the ultrasound it turned out I was only 7 weeks pregnant.
Its not uncommon for ultrasounds to be a little off on how far along you are but by 5 weeks isnt a good thing...im sorry hun but if she is truely suppose to be 10wks its not looking good...im praying that im wrong...good luck and best wishes to you and your gf