Measuring behind

So I got pregnant with IUI so I know my conception date. I had a first ultrasound at 6 weeks 4 days. I measured 6 weeks 3 days and saw a HB of 115. I went back 11 days later at what should’ve been 8 weeks and was only measuring 7 weeks 4 days, with HB of 157. Dr said she wasn’t concerned with measuring behind, even though I should’ve been 8 weeks. Just wanted to see if anyone has had similar situations and been ok? I go to my OB June 1 but it feels like an eternity away 😩