Girl offered to do my homework? I said no but she still did it, why did she do that?

Anonymous
A girl who'm I barely talk to offered to let me copy a paper but I declined because I don't like copying and teacher passes out different sheets to prevent copying. While I wasn't looking the girl took my paper and actually took the time to look in her notes for answers and wrote them down. I noticed later but didn't say anything because I was surprised. After she was done she gave it back and I thanked her and she was just like no problem. Why would a girl do this?

Did she feel sorry for me? Was she just looking to get good karma or something? Might she have a crush on me? I don't understand why anyone would just grab someone else's homework and do it.
Updates
+1 y
I've only talked to her for a bit since then because I've been sick and feel crummy enough to feel non-talkative, she's just quite too and I see her just texting.
Girl offered to do my homework? I said no but she still did it, why did she do that?
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