Scared of a new due date :(

My cycles were always crazy irregular, anywhere from 30-120 days, so when I got pregnant, there was really no telling when i ovulated. At my 7w US, my OB said I was 7w3d with a due date of 12/11. Last week, she sent me to a different US tech for my NT scan, who told me that the baby wasn't measuring that big and my due date was closer to 12/18. I was fine with it at first, but after reading some posts on here, I'm terrified that maybe that means the baby isn't growing and it will end in a miscarriage. Any thoughts?