haven't gained weight

when I was 17 weeks my doctor told me I had to grain 1/2-1 pound a week from here on out, I'm 19 weeks and still haven't gained any weight. I eat, I've changed my eating habits (I used to eat kinda crappy) and I eat better now and I very rarely let myself get hungry, I eat 3 meals a day and snack throughout the day. I run a machine so I'm standing all day but not walking around so I wouldn't think that would cause me to not gain weight. I have horses, so I do a lot of lifting and exercise in the barn (I'm not riding anymore) but I'm just worried cause I haven't gained. any tips? I have an appointment next week and I'll talk with my doctor