6 weeks and 2 days ultrasound, Only saw a yolk sac is that bad?

I went for an ultrasound on the 16/06/2011 and i am mean to be at 6 weeks and 2 days then .. now i am meant to be 6 weeks and 5 days ... all they could see was a yolk sac? what does this mean? does this mean i dont have a baby growing inside me now? or could my dates be wrong and could I be earlier than they think? the lady that was doing my ultrasound pretty much told me nothing just that it could be to early and they will prob send me back for another ultrasound in a couple of weeks Please help do i have anything to worry about?? I have no cramping or bleeding