Skinny baby

My baby has reflux so he’s not gaining weight. He finally moved from 10 percentile to 14 percentile so the doctor decided the weekly appointments are not necessary.

I went to the store yesterday and someone asked me how old he was. I said 12 weeks and she then asked me if he was a preemie bc he’s so tiny.

Should I call the doctor, I can’t tell if his size looks normal. To me he does? We are going to g.i Thursday.