How do I lose 1kg?

I'm 14 years old, weigh 8 stone 4 pounds (normally) Only yesterday and a few days ago I weighed around 54.75kgs (at least thats what my scale said). Now I ate a bit more and weigh 55.75 kgs. When I look in the mirror I think I don't look the same and my stomach is getting rounder - though everyone still says I'm skinny. My clothes still fit though, I've done some exercise but I'm still the same weight, and this brings me near tears because I think "I hate being heavier" and cry because I think my stomach used to be flatter. How do I lose this 1kg to get me back to my normal weight? I've tried everything - and I don't eat that much anyway