Why does she get so mad at me?

I currently share a with a female who is roughly my age (24) she has always been quite snappy with me but last night was different. She lost it at me. I admit I was about to cry I don't know what I did wrong to deserve that. Then when I walked past her room she had her head down and asked if I was hungry in a soft voice. I couldn't believe it. She was angry like a lion 10 minutes ago and now she's disappointed at herself and upset. I was in shock. Why does she get so mad at me. To my knowledge I haven't done anything to her. Please someone fill me in so I can rectify anything I have done wrong.
Why does she get so mad at me?
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