Why did my gf slap me after I said this?

She asked what I was doing today and I told her I went to the movies with Emily, our best friend. What I don't understand is why she slapped me. I don't see her as the jealous type and since we both share the same best friend, I really don't see why she would be upset over this.
Update: I can't ask her cause she left her phone here.
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