Take your girlfriend to a class reunion and the ending

I'm a skeptical person in love, so I advised my girlfriend at home not to go to the party anymore, but she insisted on going. She dressed very beautifully that day, so I suspected something was wrong. Already. The truth is that Mia met her ex-boyfriend at the party, they drank so much that they got drunk. Mia called me to say she would be home late but she was gone all night. For what? To fuck with my damn ex-boyfriend, why?