Small stomach?

I’ve gained around 40-50lbs. And I’m 28 weeks now. I went for a check up and my ob gyn said my belly was too small to be where I’m at but I feel it’s pretty big she said the baby seemed

Fine from his heart rate but it scared me into thinking something is wrong with my baby. She’s not doing a sonogram for another 4weeks so I’m super anxious to find out if he’s really ok.. did anyone experience something similar??