Having trouble gaining weight!

Today I had my 34 week check up. (First time mom, btw) I'm very petite.. about 5 feet tall and 100lbs before pregnancy. I'm currently weighting 116lbs. I eat alllll the time! My belly is measuring perfect at 34 weeks, but my doctor is giving me a really hard time about not gaining enough! I go back at 36 weeks and if I haven't gained anything then I have to see a nutritionist, and ultrasound to make sure baby is growing! I'm so upset about this, I have no issues eating. It seems like I'm always eating all day, everyday! Its so frustrating!

If anyone has ANY suggestions, I would love to hear them!