Try not to worry. I had my 36 week appointment yesterday and the midwife said the baby measured 6 WEEKS behind. So today it was off for an ultrasound. The measurements of all her body varied from 33 weeks to 40 weeks. The sonographer was fine with this and said all babies vary in size so this is not unusual. I seriously think it would be so much better if we didn't see anyone from say 12 weeks until birth!!! I had also been told I was breech (by 2 midwives) and found out today that is not true, she's head down. And because I know she hasn't moved position, it means the midwives were wrong.
I know it's hard not to worry, but I have found that no one has had consistent measurements (that I know) and the amount of people who've been told their baby is the wrong size is endless (and they're all born ok). Try to relax and get an u/s as soon as possible so it can put your mind at ease xoxoxo