Stomach measuring small.

Shun
I am 35+1. My doctor had told me that my baby's stomach is measuring smaller than it should be. Heartbeat and the rest of his body parts are measuring just fine except his stomach. He said he's going to wait for the third result from my ultrasound tomorrow to see what's the next step we're going to take and if it shows that he stopped growing or if he's still growing slow they might have to induce me. I asked him what I did wrong or what I could possibly do to help him grow. He's like it's probably he's not getting enough circulation from the placenta and to drink a lot of fluids and get plenty of rest. I feel like a horrible mom already.
Even if he says to not worry, it's really not easy to not worry. I get that I'm so petite and tiny that could also be the cause of it but I'm just so scared and worried :/