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1.
If a person who is usually mean the entire month, suddenly is nice for a few days and then goes back to being mean- they may have Reverse PMS.
Danny: "I was walking down the street and Brenda actually spoke and said hello to me. It sort of freaked me out."

John: "Don't worry, it is just Reverse PMS, she'll go back to being herself in just a few days."
by DOTK December 14, 2011