What should I do?

I'll call myself C but I'm a sophomore in high school and she is a junior. She is in my orchestra class. She is considered pretty popular and might be in a relationship with a guy but I'm not sure. I'm pretty average. She had never said a word to me until last Thursday. We went to state competition and our friends hung out so we ended up talking quite a bit. On Friday I noticed her looking at me and when we got to class she said hi and we talked a little bit. On Monday she smiled at me and said hi I wasn't in a good mood so I said hi but didn't really continue the conversation. On Tuesday she completely ignored me. Wednesday she ignored me until my friend walked into the room and was trying to get her attention. I told her that and after she talked to my friend, she turned around and smiled and said thanks C. On Thursday she ignored me again. On Friday I saw her in the hallway and she said "what's up C" and smiled I said hi but we were passing each other so that's really all that happened. At class though, she didn't say anything to me even with plenty of chances. I just don't understand what is going on.