Went from me crazy about me to avoiding me, WHY please help?

I dont get this girl. She was literally from the get got really into me. Following me around and we have so much in common. Asking what im doing and were im going. Talking flirtatiously with me constantly, making it obvious she wanted to be with me. Her eyes just light up when she sees me. I was sitting around my estate area with a few friends and she comes by asking if we wanted to go for a walk. No one was going so she walked on , i felt guilty and said to wait up ill go. She was delighted and gave me a high five. As i walked on i met her friend and said i had seen her before , just being friendly. The girl i like who seemed to like me looked mad at me and walked back and started talking to a guy im not that much friends with. She started flirting with him in front of me but she didnt look geniune to like him. Now she is going out with him and seemingly avoiding me like the plague. Looking like she cant stand me or wait to get away from me now. She doesnt look happy with him but i dont know what changed. I seen her looking at me and smiling with her friend talking about me yesterday then when she seen me looking she looked mad. I heard her saying also i want that *** as i walked by but kept her head down. I dont get this beahaviour at all. Does she hate me now or what. Please help.