Early ultrasound measuring smaller than I expected

Hello! I had an early ultrasound today to check out some brown spotting I've been having. Based me opk tracking, I should be 6 weeks 4 days, but baby only measured 6 weeks. There was a heartbeat visible but it was too tiny for the machine to get the rate registered.

Doctor said everything looks good and nothing to worry about, but she's not my favorite doctor so I just wanted to hear some opinions from you ladies!

They said there was a tiny SCH causing the spotting, nothing of concern