I’m still really small is my baby normal size

I’m 28 weeks on Saturday. I get my 3D ultrasound on Monday. I’m still barely showing and I’m worried that my baby is gonna be smaller than she’s supposed to be. I want her to be healthy and want good pictures of her at 3D ultrasound but idk if I should cancel it and make it for later. At my 24 week appt my belly measured at 22 weeks.