Why am I gaining weight for no reason? Please help! 10 points for best answer!?

Ive been gaining weight latley my normal is around 120 over the last few weeks Iv been about 123/124 ish. Also Im 16 and I ate lik 4000 calories, and my weight was 127, and then the next day I ate almost nothing, and then the next day I ate normal amount of calories (but I only ate at night because I was busy in the morning so I had like a 1200 calorie meal). I did excersise too! Then today in the morning before breakfast I was 125.4 which is normal and I ate breakfast(pancakes syrup, quesadilla, and refried beans) It was like brunch. Anyways now im 127.7. Why am I gaining weight so quickly I eat well and excersise. How can I change and lose weight and get back down to 120? Help please Im freaking out!