Girls, Why is it getting awkward around a girl at work?

We got along so well and talking was amazing. I could tell we really enjoyed talking with each other. She'd smile at me make eyes contact, always wave the first moment she noticed me. I did respond to those things too. Now I've noticed she barely looks at me in the eyes, doesn't wave to me anyway. I feel like something is going on between us but I can't work out what it is. But I'm still making her laugh and we are still having some conversation. But I definitely feel it's getting tense between us. There is no reason for this. She is driving me crazy.
Girls, Why is it getting awkward around a girl at work?
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