Low end of weight gain

Me

In 27wks and my baby weighs 2.5 lbs and is measuring on point. The doctor doesn't seem to worried but I'm just afraid that maybe they are missing something. I was of normal weight before pregnancy at 123 lbs. I've only gained 13lbs and they did say I'm in the low end and if I wanted to, it's ok to add in some protein shakes or bars. They only said this after I brought up my weight gain so they wouldn't even have mentioned about upping my protein intake to make up for me being on the low end. Has anyone else had this problem? Did your baby end up having a problem after all? My first pregnancy I gained 35lbs and I know each pregnancy is different but after a mc at 14 wks I'm just scared of something being wrong.