Baby Measuring 3 weeks ahead

So my OBGYN did 1 measurement when I was 6 weeks pregnant and gave me an EDD of 9/30. She never did another scan. I’ve had multiple ultrasounds done at an ultrasound clinic and they all said EDD of 9/25 as well as my anatomy scan but my OB still says 9/30. Well I had an ultrasound done today at the outside clinic and based off of all of his measurements he is measuring with an EDD of 9/7. According to my OB I’m supposed to be 33 weeks this week. according to this scan, I’m 36 weeks and he is weighing 6lbs 1oz. Has anyone ever experienced anything like this? I’m pretty confident I know when I ovulated and a due date of 9/20 is most accurate to me. But I’ve also never had a baby and feel brushed off every time I bring up measurements to my OB. I’m just terrified of going late and having a 12lb baby as I am very petite. My sisters baby was 10lb1oz.