How have I lost 10 lbs in 4 days?

I'm 5'3" and am 16 years old. Recently, I'd gained a little weight, so I decided to go on a low calorie diet using this app called "MyNetDiary" on my iPhone. I'd also read online that, for my height and age, a good weight to be at would be between 115 and 120 lbs, so I decided to aim for 118 lbs as a beginning goal and have been thinking about trying to reach 116 after I've reached it. The diet that I've been on has restricted my calorie intake to 1, 298 calories per day, and doing this was only supposed to make me lose 1 to 2 pounds per week. The lowest amount of calories that I've eaten over these past few days was 844, but I woke up later on that day and ended up skipping breakfast because of that. On every other day, I've eaten 1, 000 calories or more. The only differences in what I've been eating and drinking are that I'm drinking more water, eating slightly less, and am eating/drinking no sugar. I'm still eating carbs and have not even been exercising to lose this weight. I've been trying to diet in a healthy way that would be able to stick even once I stopped dieting. I have no idea how this happened and would like to know if anybody knows how this could have happened. At this rate, I'll reach my goal within the next few days…

Just jog and eat less and you'll lose weight. It's not rocket science.

•Do daily exercise. Exercise is necessary. Small changes to your activity level can add up over the course of a day, and can start creation an impact on your weight within as little as a week. The key to quick weight apart from diet change is exercise are Walking, cycling and swimming are the best ways to lose weight.
•Eat healthy home cooked meals, Use less oil, low salt, fresh produce, and you'll start seeing results in no time.
•Avoid the junk food like that sodas, cookies, ice cream, pizzas, and other fattening and highly processed foods.