Girls, Why is my female coworker suddenly nervous/shy towards me for no reason?

The girl I work with is suddenly nervous around me. We spoken heaps and known each other for sometime. I've noticed she is nervous/shy around me for no reason. But she still puts herself in situations where I could easily approach her, but we seem to avoid each other for no reason. But she seems happy to talk to me when we do
Girls, Why is my female coworker suddenly nervous/shy towards me for no reason?
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