Is it normal for my cousin to act like this?

Alright well I have a little 15 year old cousin named Thea and she is adorable, I love hanging out with her.

The only problem is she always has herself kind of curled in on herself. When she sits down she will lower herself, pull her knees up, and wrap her arms around them. When she lays down it's usually curled up in a ball, and when she walks she sometimes will wrap her arms around her torso. She almost never lets her body be fully exposed. It's always like she's trying to hide, and she is always kind of tense but tries to hide it.

At first I though it was a insecure thing, but she is gorgeous and very friendly and social. She has tons of friend she hangs out with, and has no problem hugging people. She even goes out on dates sometimes. But even doing that she never really lets relax.

So I am not sure if it is just a habit, or what. Do you know anyone else who will act like this? I am to nervous to actually ask her about this.