I know it must be worrying. I think the u/s have a 1 wk margin of error aswell so that could be contributing to it. I had a scan this time at 9 wks and was told I was 3 days behind and then at 13 wks apparently I was 3 days ahead (I know my dates are right b/c I know when I ovulated). It is great that you saw a heartbeat, you wouldn't be able to see that at 5wks so hopefully it's just a baby that's smaller than the 'average' they use to work out how far along you are