Why does she seem nervous/shy with me?

I've had full proper conversations with and I have known her for sometime. But for some reason she seems nervous/shy around me. Like she will happily say hello to me using my name and will wave to me. But when I talk to her she ends up playing with the collar on her shirt or lightly scratch the side of her neck. She doesn't behave like this around anyone else at work. The way she said hello to me recently by calling out my name and waving she doesn't do it with anyone else
Why does she seem nervous/shy with me?
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