Ultrasound measuring 1 week behind

Ch

I went for an ultrasound a few days ago, and according to my last menstrual cycle I should've been 9weeks 5days. However, the fetus was measured at 8weeks and 4days. Baby has a strong heartbeat, thank God. I just don't see how it's possible that I could be 8w 5d based off when I took the test my last period. Should I be worried?