Why would a girl constantly flirt with a guy if she has a boyfriend?

There is a girl who works at another location of my company and I visit to get product 2-3 days a week.
I had asked her out but she told me she had a boyfriend, and ever since she's being increasingly nice to me and I thought I misread the situation.
One of my colleagues has come with me, and she's started to see the same thing I'm seeing. Even brining up how the girl gets irritated if I don't have a long conversation with her.
Her boyfriend works at the location with her. I've tried to avoid them but she will hang around me while I'm there. Still one of the managers and my colleague has said how she's actively flirting with me. She even did it while her boyfriend was there today.

My colleague asked her about her boyfriend and she seemed very dismissive about it. My colleague said that indicates drama or something happening and I should stay away. Or she may be trying to get to know me and replace the boyfriend. But she's not sure either.

I'm not understanding why she's acting this way. As nice and fun as she is, it's also annoying how she is with me. What's her motive behind this?
Vote A
Looking to cheat
Vote B
Looking to replace
Vote C
Something else
Vote D
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Why would a girl constantly flirt with a guy if she has a boyfriend?
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