What was possibly up with her?

I went to a restaurant, and I turned around feeling someone was looking at me. And this random girl was staring at me with an angry look into my eyes. I was like “Okay… (awkwardly).” So I looked away. And she didn’t break eye contact once. Who knows for long she was staring at me? What was up with that? Never had that happened before.
What was possibly up with her?
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