Confused on due date

My last missed period would make my due date February 28th.

The first time I got an ultrasound I measured 9w1d. And that would make due date February 25th.

I know a 3 day difference isn't really a big deal but I'm confused about why when I went to my second ultrasound appointment she told me I'm 13w5d and I told her about the first place that did my ultrasound and she said my baby is measuring 14w1d but that doesn't change my due date... um what?

My first ultrasound was a transvaginal ultrasound and I only went there because they gave me a free ultrasound.

The second place I went to his my primary care doctors office they have different medical services.

Okay so which one do I go by? February 25th or 28th? I mean they both measured the same thing..

BTW sorry if I'm asking a dumb question this is my first pregnancy.