Should I call my doc?

Ana • 29, happily married, pregnant with baby #1
I am 18 weeks tomorrow (yay!), almost half way there! However, I'm concerned, but don't want to bother my doc with every question I have, hoping you mamas can provide some answers. So far I've gained 5lbs and several of my mama friends made comments as to how small my stomach is and how little weight I've gained. I've read articles that say it's extremely important to gain weight for the healthy baby, but mine seems slow on the rise. I'm not dieting, I eat healthy and plenty. Should I be worried or is more to come? Thanks!