Dates not lining up!

I had an early ultrasound 3 weeks ago for a bleed. At that time there was a heart beat and they told me I was 6 weeks. Yesterday I went in for follow up ultrasound and baby measured 8 weeks and 2 days. Heart rate 188! But It doesn't add up! I looked and for me to be 8 weeks and 2 days I would have concieved only 1 week before a positive dollar store pregnancy test. Has anyone else experienced dates not lining up? I'm concerned.