What Does Yolk Sac Being Flat Shown In Ultrasound Suggest?

I went in for my first ultrasound and my doctor said the baby is measuring at 6 weeks and 6 days but from my lmp cycle I should be at 8 weeks and 5 days and the yolk sac is flat. What could this mean? I also know what day I concieved on which was jan 5th which would put me right at 7 weeks.