Anyone having trouble gaining weight?

Brittany • 26, married, one girl and two boys.
I'm going to be 18 weeks tomorrow. I started with a normal BMI. I'm still 5 lbs under my prepregnancy weight, and gave only gained 1 lbs in the last 6 weeks. My doctors are concerned and having me see a nutritionist this week. I already eat pretty healthy (could always use a few more veggies, but who couldn't 😉), stay away from junk most of the time. So, I'm not sure what advice she can really give. I'm just curious if anyone else is having this issue? My doctor tried to tell me to drink a milk shake every day until I gain weight, but that just does not sound even slightly healthy or like the right choice to me. Thoughts?