I had an internal US at 6 weeks and could just see a little bean! These scans are much more clearer, and i knew i was exaclty 6wks (IVF). Perhaps your scan wasnt able to pick it up just yet, as you may not be as far as you think? I know when my Embryo was put in and that i was 6weeks, but the US said i was only 5wks. A mm out when measuring can make a big dif to the dates too.
I hope everything goes your way.