Measuring smaller than dates by LMP

This may be TMI, but I need some opinions. I went for my first appointment today, which was ultrasound and nurse only, no provider. I am 8 weeks 0 days by LMP, but by ultrasound, looked to be only 6 weeks. My LMP is exact, I tracked it with the glow app. I haven't had sex since 11/8. Is this worrisome that my dates are so off? Is this a sign of impending pregnancy failure? I couldn't ask the OB since this was a nurse only visit, but I'm very worried that a 2 week discrepancy, when I haven't even had sex in the right window for this pregnancy to be only 6 weeks, is a sign that this is an abnormal pregnancy. Can anyone offer any insight?

Edit, I should also add that I tested positive on 11/17, which was 9 DPO according to glow.