Lmp and conception date don't match early scan date?

I had my lmp on 16/4/11 and had sex on several occasions between 25 - 30/4/11. I was also on the mini pill but had forgotten to take two. The midwife gave me an edd of 14/1/12 but I had cramping so went for an early scan. The internal scan said I was 5 weeks (when I thought I was 7) and there was only a yolk sac and a corpus luteum. This gave me an edd of 27/1. I have to go back this Friday for a second scan but am now panicing incase something is wrong with the pregnancy. Which date should I believe and what is a corpus luteum? Thanks xx