Measuring less

Glenda • 7 years trying and my baby is finally here due 6/7/18 ❤️❤️

I got my 1 st positive 9/28 and my lmp was 8 25th so in my apps i should be around 5weeks6 days I went to my doctor today and I am measuring about 3 weeks it's just an empty sac .has this happened to anyone ??? Please please help me out !!! They said it may have stopped growing or I will miscarry please help give some advice