My best friend is mad at me and won't tell me why?

Okay. My best friend Shelby I've known for quite a while kind of turned against me one Monday morning at school. We were texting fine. We were joking around and everything but when I saw her in the morning she was completely turned off. Not looking at me, not hugging me, not sitting next to me, and just plain ignoring me. I tried to ask her in the afternoon at her lockers and I was kind of joking with her and she said "Shut up! You're annoying!" and walked away leaving me to gather my things. We usually walk together. And she's been real off. At our yearbook signing party she was my friend and all that stuff and wrote in my yearbook "I will miss you over the summer. I hope we have more classes together next year. Love, Shelby". And I thought it was odd that she put that. And she kept smiling and laughing and calling me "dude" (I'm a girl). And she showed me her new belt. Totally ignored me the last day of school. What's her problem?