One of my coworkers lost 20 pounds on nutri-system. She was always keen to tell us how much she lost everytime she lost some. Before the diet, she was a snacker. She had her bag of chips that she would periodically eat through. If you have ever seen obese people walking and eating or driving and munching, you know what I am talking about. She also had big meals because her boyfriend could cook. When she went on nutri-system, she ate smaller meals, but more frequently.
So instead of 3 big meals, replace them with 4 to 6 smaller meals. This way you are eating more frequently, and with that, your body will increase it's metabolism. Add a workout routine, whether it be jogging or basketball or weight lifting, or a combination.
The reason you may have gained weight is that you are eating less, or "under control" as you put it. When you do this, you starve the body, so it holds on to what it needs, and slows your metabolism, causing weight gain. Not to mention that you are putting off your exercises, just like your body is putting away the fat so it can use it another day.
Here's a cardio/pilates video you can go along with, it's 50 minutes long, let the ad play at the beginning.
http://www.eztakes.com/store/streaming/ ... id=0170504