Why am i not losing weight?

At school, for about a weeks, I've been going about my usual dietary routine - eating a mix of healthy and non healthy snacks between meals, reasonably healthy meals (although a fair amount of carbs) and so on half term, i went healthy and ate much healthier, less snacks, went running 5 times a week for 20 mins, and yet at the end of the week, i had GAINED weight - why??!!!! this makes me really frustrated, especially the fact that when I try, it goes the opposite way. why is my body doing this?!!