Why is my sister acting so moody lately?

My 11 year old sister has always been sort of dramatic, and moody, but this summer, she has been acting more dramatic and moody then ever. She is going into middle school, so I know she is trying to fit in, and act more grown up, but its getting really annoying! Like, this morning, I was cleaning my room, and she comes in and starts messing with my make up. Nicely, I said to get out so I can finish cleaning my room. She got really mad, through all my makeup on the floor, knocked over some other stuff off my dresser, and stormed out. I am sixteen and I have never been that moody when I was her age, I have always been a pretty laid back person. She isn't about to start her period because she hasn't said anything about cramping or anything other symptoms.