Changing Due Date??

Hi everyone!

This is my first pregnancy so everything seems weird to me but is this normal?

I found out I was pregnant on September 16th and everyone says to.use the first day of your last period to determine the due date so originally I was due May 16th. We went for my first ultrasound yesterday and they said I was actually 6 weeks not 9. They've asked me to come back next week but I'm wondering if the baby could just be smaller for some reason? We were still able to see a flicker of a heartbeat but couldn't hear it 😥 and of course it pushes back the "safe" date of being out of the first trimester. Just wondering if this has happened to anyone?