Stomach measuring small

Hey ladies. I’m hoping someone here has had experience with this and things turned out fine. I just saw my Ob this morning and I’m about to be 33 weeks on Monday! He said that my stomach is measuring too small for how far along I am and that if it still is measuring small in a month they will do more ultrasounds. He didn’t do an ultrasound today which was weird since he seemed concerned. What bothers me is that he told me to monitor my baby’s movements and to go to labor and delivery if they decrease, and that I can possibly deliver early. I’m really worried because up until now everything has been fine! I just want this baby boy to be healthy and growing! He’s my rainbow baby 😭💙 if anyone has went through this please feel free to share your story