My co-worker has very weird body language and habits in a specific situation. Why?

In the place where I work, there is a girl that shows a very specific tic in a very specific circunstance. Every time I go to talk to her, she almost instinctively grabs her phone in her hand. Then, she looks like very impatient and uninterested, as if counting the seconds for me to leave. Sometimes, she outright ignores me and starts using the phone, as if I am not there.

BUT, she only do this when she is dating. When she is single, she talks to me like a normal person. I can tell if she is seeing someone or not by this habit. Also, so far as I could observe, she only do this when I am around. With other people, she behaves normally.

It is a very specific situation and I would like to understand what causes this behaviour.

My co-worker has very weird body language and habits in a specific situation. Why?
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