Why is his girlfriend awkward around me?

My really good male friend's girlfriend is so awkward around me. I try to talk to her and smile but she is just quiet and looks almost sad and annoyed. Why is this? I know that she is social and bubbly personality.

i asked my guy friend if it's okay for her that we are friends. He said that she is fine with it.

Also, what can I do in this situation? My guy friend is really important to me and I'm important to him so I want to make things work.
Why is his girlfriend awkward around me?
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