Why does my friend do this?

I'm a sophomore in high school and I'm a guy. My one friend is a junior girl and we have orchestra together. We have grown alot closer over the past few weeks since we have had alot of competitions and concerts. We still aren't extremely close but we are at least friends now. Basically sometimes she completely ignores me in class and just talks to other people. Other days, she is completely different. On Saturday we had a competition and we were driving on a bus to get there. Once we got there and were about to perform, she started talking to me as we were sitting right next to each other (we don't usually). On the way back she kept looking back at me and smiling and doing things to get me to laugh. After we got back, a small group of like 7 people went to get food. We were both apart of that group and while we were there, everytime she laughed she would look over at me. There are other days where she does similar things and other days where she ignores me. I'm pretty sure she already has a boyfriend so I don't really think she would like me but idk. Why does she do this?