How to lose that winter weight?

I'm 16 years old, 5 foot 8, and 147 pounds. Over the course of last year, I lost about 30 pounds, going from 170 to 140, which is a healthy weight for my height. After summer 2010, as I started school again, I lost almost 5 pounds... Why would that be, do any of you know?

During this winter however, I happened to gain around 7-8 pounds. I noticed that I was gaining, so I went back to doing the same kickboxing workouts that I had used to lose weight the previous year. Nothing. It's now April, and I'm still 147. My eating habits are great - no junk, limited portions,etc - and I do cardio and some weight training 5 days a week! How can I lose that 7-8 pounds and get back to the weight that I maintained for so long?
How to lose that winter weight?
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