Why is it so hard to understand girl behavior?

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So there's this Stacy in college and she is just fucking up my head.
We started talking at my very first week in there. Always very kind, she would always say hi, shake hands and guve casual hugs everytime she saw me.
That was the closest I ever got to a girl, I am a total robot so this kind of contact was new to me.
We became closer and closer, even got to hang around in campus, we sometimes even walked with arms crossed together. I was excited, thinking to myself "whoa this os really happening? I just went from four whole years of rejection to a girl litteraly clinging on me!"
It continued until two weeks ago, when she suddenly stopped talking to me. The girl who was just the other day hugging me now won't even say good day.
It really let me down, no matter how much I tried to talk to her she wouldn't answer, it felt weird and really sad.
I was thinkong to myself what did I do wrong, and got to the conclusion that well, I'm just a loser and she funally realized it.
And yesterday, when I finally decided to accept the sad reality, Stacy started noticing me again, like all the good stuff came back, she even made me a neckassage in the middle of class!
So I'm thinking what the fuck is wrong with her, to change the mood that way. I'm completely lost, I don't even know what to think anymore, did that silence treatment really meant something?