Fetal growth

Anyone else measuring behind? I had an ultrasound done today and the last time she measured in the 50th percentile this time she's in the 36 percentile ....should I be concerned ??? I now have to get monitored once a week and get a more advanced ultrasound/measurements, also Dr. said sometimes the techs just aren't always right with their measurements that's why I'm going somewhere more advanced, opinions plz I'm very worried! I am 30 weeks along