Girls, Why will you ignore a guy once week and the next week you won't ignore him?

I like this girl, she ended a long relationship a year ago, but I don’t understand her behavior near me any more. One week she will be so glad to talk with me and looking for my attention and flirting, but the next week she can be so different and may ignore me as I’m not exist in the room. I don’t understand why she is doing this.

Despite last month for instance I felt we were closer, but really close. Suddenly she tells me more about herself, where she lives, what she was doing, what books and movies she sees. One week later she suddenly turned on 180 degrees. I felt she was ignoring me deliberately avoids saying hello to me minimal. With other men she talks freely with no problems, but only with me there is a big tension between us and I can’t do anything about it. We are not friends, I don’t meet or talk with her outside our class.

I did asked her out half a year ago and I’m not going to ask her again. Sometimes I think I get hints that she does want and like me very much. I look at the way she talks with all the other guys and it is totally different than with me. With me she is such a repressed and nervous and not allows herself to drift to the conversation.

Don’t know what to do. I think I must let her go because it makes me very sad.