Little help with this girl, please?

I'm kinda confused about this girl who I have become newly classmates with. Our professor mentioned us of a pdf of a book that would be beneficial for us and I couldn't find it myself on the Internet.

Next day, I asked guys that I sit next to (not my friends) about it and they didn't have it. So I asked this girl and she seemed pretty shy and hesitant at first. But as we talked about the book and I asked her to send me it she nodded her head to and said yes adding to it that she'll send it to me.

I got busy with other stuff the same day and texted her in the evening asking if she remembered me talking to her about it. She said 'no' then. I don't know what went wrong or why was she reacting this way. She was so much in agreement about sending it to me and talking so softly later but then she simply told a 'no' when I asked her online.

What do you think could be the matter?

Updates
+1 y
PS it happened about a week ago and I haven't bothered her since then. But two days after that incident took place i have caught her and friend staring at me but the eye contacts aren't usually long.
Little help with this girl, please?
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