I was the same way as you ore pregnancy and ever since finding out I have GD I've been out on a pregnancy approved diet that limits your carb intake. I've maintained the same weight now for five weeks which in pregnancy reality it means I've lost weight while the baby continues to gain weight. It's perfectly healthy and all you have to do is eat frequently but limit out carbs and sugars. Even this late in my pregnancy I feel much better about myself than I did months ago!
Eat healthy and stay away from bad carbs and starchy foods. Bad carbs would be white bread, white rice, basically white anything lol, soda, sugary stuff, fried stuff, and try not to eat starchy veggies such as potatos. Eat white lean meats, lots of veggies, fruits, and all the water you can drink. You can go for walks, and do different kinds of squats and lunges, and even lift light hand weights. Go on youtube and try the prenatal workouts on there. You can also do yoga! Its so relaxing and all it helps with back pain and hip pain a lot! There's a lot of things to keep your weight in control but don't try losing weight while pregnant. Good luck hun.