How do people switch up on you or change so quickly?

I used to talk to this girl online for about a month and a half. We liked each other everything was going good. We talked everyday morning through like 2am. We would talk on the phone 2 times a week for about 4-5 hours. This all ended when i sent her a clingy text a few weeks ago asking her if she really wanted to talk to me and what not. She replied back with an insensitive text and said she can't talk because like there is only one app we talk on when that isn't true. After the argument we didn't talk for 3 days until I texted her and we talked for 4 or 5 days straight but not like we used to talk and then it kinda just died down. She would start leaving me on read. August 27th was her brithday I sent her a brithday text she screenshotted it and posted it on her fb. This is odd because well I'm blocked off fb and can't see it but my Freind saw it. I asked her Why she posted it she said, "bruh". The same night she said, "goodnignt" and the following morning she said, "goodmorning". Now like things have just gone cold. She's gone Ice cold. I've tried texting her but she would leave me on read and now she unfreinded me on snapchat so I can't text her. I texted her on another app we talked off of and asked her how she's doimg and that I miss her she saw it didn't reply. Yesterday my Freind saw this post on her facebook (pic below). I think it might be about me because 1. She knows I want her by my actions 2. She's ignoring me I don't know why. How did she change so fast?
How do people switch up on you or change so quickly??
How do people switch up on you or change so quickly?
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