Why is she angry at me?

So, we spoke yesterday, and we were fine. Today, she came in for her shift visibly angry at me. It turns out I bought a gift for a colleague whose birthday is today. I only recently learned that she also has a birthday, probably in March, because we were discussing work birthday gift vouchers. I completely forgot and didn't know, but the mention of it reminded me that I bought her a gift around this time last year. The colleague posted on her status, indirectly thanking me in a not-so-subtle way, and I think she didn't like that. Why is she reacting this way? We aren't dating, and she constantly says I am delusional about "us." So why her reaction? I didn't confront her. Luckily, I can read people quickly. Just to confirm, I said goodbye aloud to her. Lol, it wasn't a nice one 😂 She said it angrily.

Why is she angry at me?
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