Amnio and really scared

I’m week 31, and up until week 26 everything was fine but then doctor noticed that my baby was growing really slowly, I did the NIPT screening and it came out normal, so I was really happy. Ultrasounds show that my baby boy has developed normally and that there are no clear abnormalities but for the weight. He is perfectly proportioned but way too small. So now my doc wants me to get an amnio done after all! I am devastated and I could cry all the time because my doc can’t tell me what’s wrong :( has anyone been in a similar situation?