Hi mama! Ang dating babago ng dates based sa ultrasound ay normal talaga, lalo na from early pregnancy scan to later ones, kasi mas accurate ang dating sa early weeks. A few of my mum friends din nag-experience nito, and their doctors explained na it's all about how baby's measuring at that point in time.
But honestly, best mo i-clarify ito directly sa OB mo para sure, baka may valid reason bakit nag-adjust, or baka need lang ng explanation para peace of mind mo. Don't worry, it happens!