Why does this girl who likes my crush watch me and try to intimidate me?

She met my crush in college and has group assignments with him and a few other students. My crush used to be interested in me but we never really talked and we never sit together. I think he is getting closer to her since she is more extroverted and flirty with him but he seems to like both of us to some extent since he mirrors both our body language sometimes and glances at me occasionally. For some reason this girl from his group glances at me during the lecture and one time chose to sit right in front of me even though we were the only two people in a huge room. I feel like she tries to mark her territory but I don’t understand how she could have understood that there was something between us.

Why does this girl who likes my crush watch me and try to intimidate me?
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