What's up with this girl?

There's this girl a little younger than me (I'm 15, she's 13) that I kinda know. I notice she looks at me alot. When I turn to see her, she whips her head around quick. Sometimes she smiles at me. We talked more when we first met. Now we don't talk much, but when we do, she plays with her fingers or if we sit next to eachother, she scoots close to me. What's her problem? Why does she act like this?