Why act like this if you aren't interested?

This girl that I've known for 3 years now used to like me when we first met but I was interested in another girl so I didn't even try to think about her and I rejected her in a nice way and we stayed friends. Now we talk almost everyday and I started to like her. She gives me the idea that she still likes me from the way she texts and how she acts when we go out sometimes. She texts first , she ends her sentences in multiple letters and even asks why i don't text if i don't text her for more than 3 days. I flirted with her once and she asked if I was flirting I said no because it seemed like she didn't like it and she said I would've liked if you said yes. She did tell me about another boy she liked a lot though but I haven't thought much of it. I decided to tell her I liked her and then she said she couldn't believe it and thought I was joking but she then told me that she liked this other guy and that even if he wasn't there she can't like me like she used to anymore. I told her I understand and that I'll get over it since it's a small crush nothing serious (it isn't I am deeply in love with her). We still talk and she still has the same energy and vibe so I don't understand. Why treat me like this if you don't like me? I have other female friends and there's a huge difference between how she talks to me and how other girls talk to me.(yes there's also a difference in how I talk to her but she doesn't know that) So if anyone can tell me what's happening here please do.

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We talked today and she told me she thinks I've changed a lot since we first met. She said I'm too non chalant these days. I said " I don't think I've changed but I'll try to be nicer I guess" She then brought up the fact that I like her and said if I wanted to get over her we should stop talking. I told her that I'm already over her and she laughed and said she didn't believe me but if I am really telling the truth we can be friends, even though she thinks I'm lying.
Why act like this if you aren't interested?
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