Why would a girl give a heavy sigh when you walk by her?

This has happened multiple times and this girl is a co worker. She's a quiet girl and when I first met her she was shy but eventually she became normal. I've been really nice to her by helping her with her work because I think she tries really hard and people tend to ignore her so I always say hi and bye to her. These sighs she's been giving when I'm around are very loud and seems like odd behavior. But in the past couple of weeks she's been getting shy around me again and seems nervous around me. It makes me feel like a creep because I've been trying to be friends with her.
Why would a girl give a heavy sigh when you walk by her?
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