The same thing happened to me. My dr told me that I prob just ovulated later then most people. If you didn't ovulate on I think its day 14 of a normal cycle it would change your date. My dates where 10 dates apart. my lmp was nov 2 which would of put me at aug 10th but baby measured as if my lmp was nov 13. It all depends on the ovulation date which no one can be certain on which day they ovulated only around what time they did.
I had the same problem I thought I was about 10 works and so did the doctor but He send me for an ultrasound Cz I was really big and he thought I wad having twins but turned out I was only 8 weeks and only carrying 1 baby but lots of water. This scan said I was due on the 18th december had another can at 11w and was told I'm dud on the 19th and then had a scan at 12 weeks and showed I'm fud 21st december lol. I bot a scan monday anc hoping that its the same date. And ur doctor would have calculated Ur pregnancy from 1st day of Ur last period but you dont ovulate till 2 weeks after Ur period so this is y there is a difference
Early ultrasounds are usually pretty accurate. Good luck to u.