Whatever the doctor tells you go by that. Don't go by your estimate.
I didnt know until I was 6 weeks when the sickness kicked in! When i found out i was, was the same day i found out i gained two pounds. At seven weeks i gained four pounds! Your body doesnt always make noticeble changes in the very beginning. It is possible that your dates are off, but thats very common until you get your ultrasound its hard to tell exactly how far you are! Some women take even longer to find out! Its fine :)