Small Gestational Sac

So I thought I was 9 weeks pregnant. But I went to the OBGYN yesterday and they said I’m only 6 weeks and 4 days. Which okay .. not worried about but then the doctor was telling me that my gestational sac is smaller than normal. He said don’t worry about it because he’s seen it a thousand times and usually everything is okay. But I can’t help but worry and wonder why. He wants me to come back in 2 weeks to keep an eye on it. Has anyone had this happen? The baby’s heart beat was good (135bpm)