You can gain back a lot more than you lose, even if you get into a habit of "better" eating and more exercise for months.
You probably think that exercising and eating "better" should be fine, because anything you do for three weeks becomes a habit, right? Well, not exactly. Bad habits can be much harder to break, and you may not have really broken them the way you think you have: http://science.howstuffworks.com/life/form-a-habit.htm
Be really careful before a drastic change to your diet and exercise habits just to lose 10-15 lbs, especially if your doctor and the majority of those close to you don't think you need to lose weight. It can backfire.
However, a gradual change, especially when you can do it in a reasonable, sustainable way, can be a great idea.
You probably think that exercising and eating "better" should be fine, because anything you do for three weeks becomes a habit, right? Well, not exactly. Bad habits can be much harder to break, and you may not have really broken them the way you think you have: http://science.howstuffworks.com/life/form-a-habit.htm
Be really careful before a drastic change to your diet and exercise habits just to lose 10-15 lbs, especially if your doctor and the majority of those close to you don't think you need to lose weight. It can backfire.
However, a gradual change, especially when you can do it in a reasonable, sustainable way, can be a great idea.