First ultrasound

I just had my first ultrasound today.

I don’t quite understand them?

At my appointment, they said I’m 9w2d

My boyfriend and I first had sex on Dec14th.

So, was the ultrasound tech, wrong?

My boyfriend is the only person I’ve slept with, so it can’t be anyone else’s, but I’ve been told I’m 9 weeks now?

Dec14th was, what.. 7-8 weeks ago?

Idk, I just don’t really understand? Probably seems like a dumb question...