Estimated 9 weeks now only 6?

So based on my cycle I was estimated to be at about 9 weeks however I went in for my first ultrasound last week and the DR. Said I only looked maybe 6 weeks and said he couldn’t see much. My cycles have be irregular but should I be worried that he didn’t see much? I mean the sac was definitely there, but I’m worried.

This is my first pregnancy

Has this happened to anyone and baby was fine??