First Ultrasound at 6 weeks 2 days

Hey all! I had my first ultrasound at what I thought would be 6 weeks and 2 days based on my last period/ovulation date. I’ve been having some spotting the last few weeks, nothing extensive just light brown and only when I would wipe definitely not enough to fill a pad. My OB didn’t seem too concerned but suggested I go in for an early ultrasound just to make sure there wasn’t anything funky going on. The baby is exactly where it should be however they were measuring it at 5 weeks 5 days approximately. (Which mathematically doesn’t really make sense based on when I know we conceived but I know the human body can do magical things 🤷🏽‍♀️). The ultrasound tech was unable to get a good measurement of the baby because it was so small, she said quite literally that if I would’ve come a day sooner she probably wouldn’t have been able to pick up anything at all l. We were able to see the flicker of the heartbeat but again it was too small for her to make any accurate measurements. When she tried the heart rate she was picking up was 87. She did not seem too concerned based on the size and I have another ultrasound scheduled for the 30th. Just curious, has anyone else had a similar experience? I try not to worry but that’s easier said than done for sure!