That happened to me already. Last month, around August 21st, I was told I was about 17 weeks and 2 days, but the lady who did the ultrasound said it was only 16 weeks and 5 days. Then yesterday, I had another ultrasound because of supposed complications(there was nothing wrong, thankfully), and I was told I was 20 weeks and 4 days, but the baby was measuring 19 weeks and 5 days. However, the doctor told me if there is less than a 10 day difference, they aren't too worried.
As for your dates, though, I'm really not sure what to tell you. I'd maybe ask what could make you/the baby measure so far off based on the two ultrasounds. That is a bit confusing, and I wish I could offer more advice. Definitely keep on your doctor about the growth of your baby and things of that nature. That's all I can really think to do. Good luck!