I can't say if you were pregnant or not, but the doctor will use that as the first day of your last period to work out an estimated date of delivery for you.
Ultrasound is used in conjunction with your dates and not just on its own.
If the 2 don't match up, then they will query it.
So you've got less than 2 weeks to your scan - which would be about right - around 10 weeks.
Good luck and keep us posted.