Why does a coworker act shy around me and stare at me?

I have a coworker that for the past couple of weeks, has been acting differently around me. When she talks to everyone else, she is very talkative about work and everything outside of work. When she talks to me, she becomes very nervous and awkward. She struggles to keep a basic conversation with me. She will become so nervous and awkward that she will walk away and pretend to work. When she is talking to someone and I walk pass by she will keep the conversation with that person but stare at me and smile. When she has the chance she will find a way to walk past by me and look at me and smile. When she is working near me, she will stare at me. When I walk in a room she will stare at me. I am the only person she is like this with. I don't know what to do. She is being very shy and awkward around me. I don't know if I should wait for her to say or do something or I should say or do something first. I don't know If I should approach her when she smiles and looks at me, I dont have an idea what to say if I should. I don't know if I should wave and smile back.
Why does a coworker act shy around me and stare at me?
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