Only 3 weeks pregnant?

Reese • 👼🏼 2014 💙 Landon 2/2016 👼🏼 2018 💖 Savannah 8/2019

I got a positive test last week 1/21. My LMP was 12/19 and according to my app i would be around 5 weeks or more pregnant at this point. My blood work still hadn’t reached my doc so they just had me come in for an ultrasound to see how far along I am.

She went in and saw just the gestational sac, a tiny dot and said I was barely pregnant around 3 weeks. Is this possible?! I would assume I would be farther along.