I thought I was 8 weeks.

So had my first prenatal appt today from my last lmp, I was to be 8weeks exactly. Well went in today did a transvaginal ultrasound, and they said I'm super early about 5 weeks. Too small to measure. So assuming I ovulated super late. How was I 3 weeks off? Has anyone experienced this. I was kinda sad I thought I was gonna be able to see something bigger. I mean we did see a flicker of a bean. But I was so prepared to hear a heart beat...so I go back in two weeks to do another ultrasound make sure everything is growing... This 2 weeks is gonna be killer. I was hoping I was getting closer to being out the woods. Ready to be past 12 weeks