I went through the same thing about a month ago. My doctors measured me smaller and scheduled me for a ultrasound. Come to find out he measured big with the tech. The doctor at the ultrasound concluded that my sons head was down in my pelvis and that's why my ob couldn't measure accurate. I can tell you that I worried up until the ultrasound date. Try not to stress. Make a list of questions to ask tool prepare just in case. At the end of the day your baby will be fine. Keep the faith.
I measured small with my first pregnancy. Two weeks behind consistently so had to have a growth scan and he was just fine born at 39 weeks 4 days he was 6 lbs and 11 oz.
With my first she measured small through the whole pregnancy but was very healthy when she was born at 6 lbs 1 oz. Try not to stress about it. Good luck!