Probably a stupid question

I just had my ultrasound and they went over my results with me. This is my 3rd pregnancy but i have questions.

Ok so I’m 21w4d today but my son is measuring 20w6d. He’s in the 19th percent tile, which means he’s small according to the doctor. Everything else looked perfect, his fluid levels, and all his anatomy 👍🏼

I’m just upset, I feel like it’s my fault he’s behind/ growing small. I’m healthy and I’ve been doing everything I’m supposed to do as far as prenatal vitamins and eating right and I’m not over weight.

My question is, my due date is changed on my paperwork but the doctor didn’t say anything in person.

Do I go based off that, or my original due date still? If so, do I change it on the app too?

Just confused 😐