Serious question

I just had my 37 week ultrasound and baby boy only weighs 5 pounds 7oz. They checked everything and everything is normal he is just tiny. The doctor wants me to go back next Tuesday for another ultrasound to just keep an eye on him. He's a very active baby. But I'm so worried why is he so small? The doctor checked me and said I'm 80% and 1/2cm and the baby's head is really low in my pelvis. Anyone else who went through this? I'm a first time mom and I eat but I feel so guilty about my baby being tiny :( I just want to cry.