Small baby?

I went in and got my first ultrasound on Wednesday (my hcg levels were above 7000) and we saw our kiddo for the first time! They were able to get a heart rate (127) but they weren't able to give us an estimated due date because the baby was too small.

I'm supposed to be 8 weeks today according to LMP, and I'm kind of freaking out that something might be wrong - they said that the sac looked the right size for 7 weeks but the baby was the size of a grain of rice. I didn't get to talk to my obgyn at all, just the ultrasound technician, and she couldn't tell us anything, and we called the doctor Friday morning and he hasn't called back yet.

Anyone else experience a smaller than normal baby at their first ultrasound? Is this somewhat normal? First time pregnancy and super nervous about everything going ok!