Why is a girl who rejected me mad that I was talking to another girl?

So there was this girl I was in love with for over a year, who kept rejecting me. So finally I decided instead of going nuts about her I will move on.

So today at work, another girl was flirting with me and my old crush was furious, and giving me a dirty look. Why would she care if she didn’t want me before? People say I’m a really good looking guy, but my old crush is too if that matters.

Why would she get mad another girl is talking to me?
Why is a girl who rejected me mad that I was talking to another girl?
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