Happened with me too , plus my uterus is titled so they couldn't see but I came back the 7th week of course it was very little but I was at least able to see what looked like a little peanut with a heart beat lol
OK well that's more of a concern. Usually women are basing their pregnancy on LMP and that's not reliable but if the sac size is indicating you are 6w but no fetal pole is visible that is an issue. What did your Dr suggest? Have they done hcg draw?
I had a sonogram done at 6w with my son and they got a heartbeat
This time the sack to embryo ratio was way off
And I only had sex once the month I got pregnant and it was 5 days after my period ended
If you're less than 6w they won't even be able to see the baby - just the sac, and if you're less than 5w they won't even see a sac. Chances are you're not 6 weeks yet.
You could just be not as far along as you think. Dont stress. And stay positive. Good luck