It is definitely possible that you are not as far along as you were thinking. Many women ovulate later than expected and then get pregnant from those cycles. I think the main question to ask here is when did you get your first BFP? If it was right when you should have first missed your period, then that would lend to the idea that your dates are right and the pregnancy hasn't progressed as much as it should. However, if it was even a few days off, things could still be okay. By the end of the 6th week you should definitely be seeing all the structures and a heartbeat. But at the beginning of the 6th week it is still up in the air.
I had an early scan at 5w6d. (This was correct according to my dates and then confirmed by the measurements of the gestational sac.) At that scan, all we saw was a perfect-- but empty-- gestational sac. No yolk, no fetal pole, and no heartbeat. I'm guessing it would have been at least a couple of days before that stuff showed up. I can't be certain when, though, because I didn't have another scan until I was 11 weeks along. :-/