Help please

I had irregular periods so my dr sent me for a dating scan. I went to the dating scan and they told me I was 6 weeks today... But I just don't see how that works if I count back to when I got a positive pregnancy test, if we're talking technically I got a positive test at just 3 days pregnant, that just doesn't seem possible to me (I had to wait ages for this US) and tech is saying conception date would be around the 2nd of December (again talking technically as Drs add on 2 weeks at conception date) and I got a positive test the 5th December!