Why is my girlfriend mad over something so small?

My girlfriend and I were in bed watching the lion king. All of a sudden when simba did she started crying. I asked her what was wrong she started crying harder. Then she got mad at me for not holding her while she cried. When I tried to tell her I'm sorry and give her a hug she pushed me away. I don't know why she crying and she want tell me what's wrong. Why is so crying over a movie we've seen so many times
Why is my girlfriend mad over something so small?
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