Ahh help!! Will I gain weight?

Im 15, and im 5'1, and I just got back from a 4 day summer camp today. Before the camp I weighed 113 lbs. I was pretty active alot everyday and I barely ate anything, just because the camp food was terrible lol and I just wasn't very hungry. I think I had about 400 calories per day... I know that's VERYY unhealthy, but I figured 4 days wouldn't do that much. Anyway, I just got home and put on some shorts that wer tight on me before I left, and now their kinda loose. I have been trying very hard to lose weight(without being successful) and I'm really glad my shorts fit me and my clothes are looser, but I also know that 400 calories aren't enough and if I plan on eating 1500 calories a day, I'll just end up gaining MORE weight. I have been trying for months to lose weight and it just wasn't working, but I think I'm starting a new diet and exercise program tomorrow to eat 1500 calories and burn 2000 calories a day. That's good, right? Well Im afraid I'm going to gain that weight back because I was in starvation Mode, so does anyone know a way to start eating healthy again without gaining the weight back??