Measuring small

So I've measured small for quite a while (fundal height). The dr wasn't concerned initially since I was only a couple weeks behind. I had an ultrasound 2 weeks ago at 33 weeks for other reasons and baby was measuring normally then. But When I saw my OB yesterday, I was measuring 6 weeks behind. He's sending me for another ultrasound in a week so naturally I'm stressing. Anyone measured that far behind and still had a baby that was the normal size?? Just wanting to hear other people's experiences with this.