I don't understand girls?

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I'm a guy, and I was close friends with a girl, and we used to talk every day, and I felt like the friendship had gotten really strong. but recently she's suddenly stopped talking to me and she seems really disinterested and annoyed when i try, and nothing ever happened so i don't understand how we could have gone from so good friends to her not even wanting to say a word to me like this
i also noticed that shes talking more and more to this other guy, and she seems really interested in him fsr even though we share loads more interests. I'm pretty sure now she has a crush on him, could that have anything to do with why she's treating me like this? and if so could someone please explain why to me, because I feel really sad and betrayed because I've lost a really really good friend and I'm wondering if its got anything to do with that I'm a guy? I don't understand girls to well so it would be great if someone could help me understand