26 weeks no weight gain!! worried about gestational diabetes

So I am 26 weeks pregnant and I have not gained any weight in this pregnancy. I'm 5'6 and wear size 8 or 10 depending on brand. I'm wearing maternity pants and my tummy has a shape. It's actually measuring exact on time. I'm worried as first 18 weeks I was very sick with nausea and vomiting and lost 2 pounds that I have not gained. I have no sweet cravings and I'm walking 30-45 min a day. I eat whole grain and brown rice only. I probably had pizza twice and junk food 2-3 times in whole pregnancy. I'm worried I might have GD as I am not gaining weight. My doctor is not at all bothered and said now I will gain weight. Anyone else had any issues like that ??