Early abdominal scan. Measuring behind please help.

I know normally the first ultrasound is transvaginal well where I live you need a referral for an ob so I went to a clinic that does pregnancy testing well they give an ultrasound if you have a positive test they told me before they usually wait till week 7 and given my last period I was 6w6d could be less of course well they did the abdominal scan anyways and she said I was 4 weeks after looking and decided for me to come back for another free ultrasound. My question is should I be worried? I don’t know how I could just be 4 weeks when 2 weeks ago I was testing positive. I had a miscarriage in 2017 where baby didn’t grow after 5 weeks and I’m nervous for the situation to happen again. Anyone else have a abdominal scan at 6 weeks and they said you only looked 4 weeks but everything was fine?

Edit** no measuring done it was a quick look and done. (Not an actual Obgyn clinic it was a free clinic that does verification of pregnancy)