I don't understand my friend?

I am a sophomore in high school and one of my friends is a junior girl. We met each other this year which we have orchestra together. We've grown alot closer over the past few weeks as we've had alot of orchestra events and concerts. Sometimes in class she says hi to me and smiles and sometimes she completely ignores me and talks to other people. She looks at me all the time, for example when we were on our bus back from an orchestra contest, she kept looking at me and smiling and doing things to make me laugh. We went out to eat with a bunch of people from our orchestra class and there were 7 of us there. Our group talked about stupid things and we were laughing alot. Everytime she would laugh she would look over at me. She has a boyfriend so I don't know that anything like that would be happening I'm just not sure why some days she makes it seem like we are really close and why she is laughing and always looking at me. And then other days she acts like I don't exist.