Why was my coworker being extremely possessive?

Helloeriss01
I had a coworker but she was fun and would make dirty jokes outside of work. She was 22 I’m 21. At first it was great and we would talk a lot. I started call my boyfriend a lot at work while we were closing the store and she would have a problem with it and tell me to hang up. Now looking back in hindsight it was inappropriate. But she felt bad because she gave me a bag from the job. She did it again a few weeks later and I called him after we finished closing duties and she flat out kept telling me to hang up which was so weird. Even when we were leaving for the night and I was clocked out she was like “hang up until were completely out of the store” and she sounded mad.
She told on me after that night to other managers and they had to talk to me but they took my side. I was furious at her for months and she kept trying to make it up to me. I don't know if it was because my boyfriend or not but I heard when she goes out she always has to find a guy. Plus once I was next to her and she put her texts out right in front of me where I could see. Plus she gets on the phone at work. A couple of times after that she told me that she “cared” about me.
She decided to leave the job for a while and wanted to take goodbye pics with everyone individually. That day my manager told me the girl wanted to take a pic with me but was scared I was gonna say no because she thought I hated her. I was telling my manager about another girl from a different job who was gonna come visit me and just at that moment my supervisor walked over and asked to take the picture with me.
She unexpectedly held out her arms and grabbed my waist and pulled me in closer and hugged me in our pic. She posted her pics with everyone and she only hugged me and no one else. She kept leaving notes about how she wanted to come back to the job.
She eventually got a boyfriend and bought him in to meet everyone and she wouldn’t make eye contact with me and didn’t introduce him to me. Why?
Why was my coworker being extremely possessive?
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