I do that .. more than I like to.
But there are two guys that I mainly do that to.
1. The friend: He's so sweet and nice and we have really great conversations. But there is zero attraction to him so he is strictly just a friend. But sometime he can get VERY annoying because he is always with me and he wants to take the same classes and hang out, etc. That's when I turn cold and I don't talk to him or avoid him - this could be interpreted as nervous/shy? But when I'm not so sick of him, I'm usually very nice to him and even sometimes flirty.
2. The friend... that I like: I flirt with him. Then I realize we're just friends so I stop talking and just.. become frustrated.
So, hopefully, in your case, the girl thinks of you like I think of my number2. Maybe she'll give more hints. When she's shy and nervous, etc, try putting your arm around her. If she leans in towards you or even rests her hand on your tummy, or place her head on your shoulder, she's probably into you. But if she stays absolutely still and you feel like you just put your arm around a robot, then chances are... she probably doesn't feel comfortable with anything more than being friends.
I don't know if I even answered your question! haha
But best of luck,