baby measuring small

so I am 33 weeks and 6 days. I went to my ob today for a routine checkup. I told him that my Crohn's has been flaring up. so other things got brought up because of my Crohn's being flared up such as some pretty bad leg pains and my hands shaking. So he did some blood work just to make sure we aren't missing anything. onto the next part of the check up and he informed me the baby was measuring small. she has measured normal at every other appointment but this one she is measuring small so we have an ultrasound set up. so I just wanted to ask other mommies, has anyone had their baby measuring small? they told me not to worry and that it's probably just my body type? what types of food could I eat to help her gain weight? also does anyone else deal with having an autoimmune disease? or is anyone else dealing with bad leg pains and/or shaking?