I'm a little behind on pregnancy..

exactly a week ago, I got an ultrasound and they said I was approx. 6 weeks 1 day and today I went back and they said 6 weeks and 6 days..heartbeat was there and it went from 103 last week to 122 this week, so the heart is fine and everything else looks good but is my baby not growing to where it should be? They think I could've ovulated later, my last period was July 25 and I prob ovulated around aug 8-9. Has anyone has this happen to them and baby turned out ok?