Measuring 4 weeks at 6

Hi Ladies, I am having serious anxiety after having my first ultrasound. I am 99% certain I am at least 6 weeks (even 6 weeks and 4 days), however, my scan is showing a gestational sac measuring 4 weeks. How is it possible to be just 4 weeks if I even had a Bfp on 8/31? My doctor told me to come back in 2 weeks. No blood was drawn and I just feel like I'm out. I also had a chemical pregnancy in June and have struggled with infertility for years. Anyone have no heartbeat and smaller sac at 6 weeks but saw something at 8?