I think, from personal experience of having 2 kids, that the abdominal muscles in the stomach get pretty loose or just weak after pregnancies. I'm thin, work out a lot, and my stomach isn't what it was pre kids. But I also don't do much to help that area of my body. If I did, I think it would be better. I especially noticed this after my second child. I never had to focus much on my stomach area before kids. I have heard that holding your stomach in while sitting, like when you're driving a car, can help to strengthen those muscles. I even think they make a band that you wear around your torso that's supposed to help strengthen that area. Then, there's always crunches, push-ups, leg lifts...strengthening exercises that can help. But, none of that will really make a noticeable difference without a good diet. Eat healthy and monitor your calorie intake, losing excess fat in conjunction with exercises will get you results. Try a calorie tracker like Livestrong My Plate, just to get an idea of what your calorie intake should be and what you're taking in. It was a little eye opening for me when I first started using it. And, at 5'7" 140 lbs, your BMI is 21 which is completely in the healthy range, so I doubt your belly is as big as you think it is. But, good for you for trying to make some healthy changes!