confused! just had early scan...

Judging by my LMP I had calculated I was 7weeks 4 days, obviously dependant on when I ovulated and conceived. However after having a transvaginal U/S the sonographer found the yolk sack and mini heart beat. She however thought that I was earlier than 7w4d due to the fact the fetus was only measuring 2.5mm which is more like a 5w sized fetus. I now don't know what to think! Does this mean I am 7w4d and the baby isn't growing at the right rate and we are likely to MC or does it mean that we conceived 2 weeks later and the fetus is the right size for 5w and we are lucky to see a heartbeat?! Booked in for a re-scan in 2 weeks but just don't know how to feel?! Help!