Measuring behind on both ultrasounds 🙃

Okay so I went for my first ultrasound (abdominal) when I was supposedly 7 weeks and she could only see the sac and said I was measuring 5 weeks 2 days so come back in two weeks to make sure baby is growing. Well I went back yesterday when I should have been 7 weeks 1 day based off last ultrasound and we saw baby with a heart beat of 113 but I was only measuring 6 weeks so the tech said I was probably around 4 weeks at the last ultrasound. ( she did another abdominal scan) I’m so happy to see baby had a heartbeat but can’t help worry that I continuously measure behind. Has anyone experienced this? And I’m also confused because I can already feel my uterus when I lay flat which I couldn’t feel till I was between 11-14 weeks with my previous pregnancies. This will be my third baby. I go back in 3 weeks to make sure things are progressing as they should