Why does my female friend seem jealous?

I'm a single man, and I've been friends with my engaged friend for around 5 years now. I've supported her in a mairrage, helped her get through a divorce, and am supporting her in her new mairrage. She's currently engaged, but we talk almost every time we are together, which is all the time since we're coworkers. She loves to be around me because she's always seeking me out. But my crush is working with us again, and my friend has shown support in me pursuing my crush, but she hates when I talk about her. She starts getting distant and saying little to nothing, and when I try to ask her for advice she just gives dry answers like she'd rather not see me with her. My friend is in a very committed and serious engagement, but I feel like she doesn't want me to be another girl. Anyone know what may be happening?
Why does my female friend seem jealous?
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