When did you start gaining weight?

So I'm 15 weeks and I've gained a total of 3 lbs since before I was pregnant. In the first trimester I had bad morning sickness but I never threw up, I actually ate constantly to try to feel better. My appetite has been normal and I've been eating way more than I did before becoming pregnant. Mainly carbs. I don't count calories but i feel like I eat a ton and all the time. I eat more carbs and Calorie dense things now than before I was pregnant. I've actually lost a couple lbs since my last appointment. I would understand it if I had been throwing up a lot but I haven't. My doctor wants me to gain a lot because I'm tall and slender but I feel I have been doing everything I can think of to gain more weight. I had an appt today and we listened to the heart beat again and my doctor said everything seems fine as long as I start to gain. Has anyone had this problem?