Measuring a week ahead

Alicia • Got married Nov. 8th, found out I was pregnant 12/9... It`s been a crazy and awesome month
I just had a ultrasound today and I'm 39 weeks 4 days but my baby is measuring 31 weeks 3 days. The Dr. who reviewed my ultrasound said he wanted me to come back in 4-6 weeks to measure the growth. I'm worried something's wrong, even though he said everything looked good.