Why is she flirty one day and then not so much the next?

In class she sits next to me all the time. She's really nice to me everyday, it's not like she ignores me some days and then not others.

But I'm the one always looking at her and touching her arm and teasing her in class. She lets me touch her and she lets me get close to her.

The only time she's flirted back is when we were together studying for a test. She wore some revealing clothing, sat right next to me as opposed to across me at the table, and "accidentally" brushed my hands and arm, legs throughout the night. She was constantly fixing her clothing and looking at me and she seemed to want to be flirty back with me.

Then I see her again and she just treats me like a friend, just being nice. How do I know when she's not flirting for attention or fun, or if she actually likes me? I mean one day she'll be "accidentally" touching me a lot and getting close, and another day she'll treat me just like a friend, be nice but not touch me or anything. How do I figure out what she wants?
Why is she flirty one day and then not so much the next?
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