Fetus measuring small..any experiences??

Finally had first doctor appointment and left there as confused about gestational age and paternity as I was when I went in.

The ultra sound tech said I'm very early.. she said around six weeks, but that the embryo is only 3mm in size. From what I've read and seen so far I'm very confused if those two things are accurate?

Any one else have a similar experience? I have irregular periods so I'm confused how accurate this first image really is. When I was six weeks with my six year old twins I could see them much clearer

This image today I could barely even see the embryo...