Maybe you were stressed or something. You will have to try eating healthily and excercising more but with exercise you have to do it slowly and gradually at your own pace, don't do it to fast though.
Hypo has some good suggestions but I would suggest seeing a doctor to just to rule out diabetes type 1 and thyroid issues too. Your weight is being constant, and your symptoms could easily come from only eating "junk" foods. However, both are serious conditions that need to be ruled out, especially if you're underweight.
Usually at that age is when Human Growth hormones are at there peak. HGH stimulates skeletal mucle and bone growth and well as using fat a a primary fuel source, but you need to eat protein in order to build msucle, calcium and phosphorus, B12, folic acid, to build bones. So if your night eating actual "food" food your body has a hard recognizing it as fuel or to activate hormones. Wrong kind or no fat depresses the endocrine system, wrong kind of carbs can make you resistant, diabetes , etc, and you won't grow without protein(or make new red blood cells so you brain won't get oxygen, no muscle tisse, no amino acid hormones, etc) try eating healthy foods and you'll notice the difference in your energy level and development.