First scan dating two weeks behind my dates (help!)

Hi,

I’m looking for some advice. Today I went for an early scan after some brown spotting for over a week. I have two children already and had the same brown spotting with my youngest and the pregnancy was fine. 

At the scan today I estimated I was 8 weeks + due to having my last lmp at the end of Feb. The sonographer said the baby was measuring at 6 weeks as was the sac. She suggested my dates could be off and to come back in two weeks.

I didn’t feel my dates could be off as I know when i conceived and had a positive pregnancy test on 27th March. 

Any advice or ladies in a similar situation?.