Cady was holding her party on the same night that Janis Ian would be featured in an art show. Cady herself was also featured in that art show and won a prize in it. Regina George did not get invited, much to her disappointment. Janis Ian was disappointed to not be invited as well.
After Gretchen and Karen started following her around, Cady was forced to stay home and eventually decided to throw a small get-together at her house. However, as the word about her 'party' passed through the entire school, her house was soon-after filled with dozens of people she barely even knew.
Shortly after, Cady got drunk so she went upstairs to pee. Unable to think straight, she passed out in her bedroom. Eventually, Aaron Samuels showed up, and the two had a conversation. While these events were taking place, Regina and Shane Oman arrive at the party, and start to search for Cady. After Aaron realized that Cady's behaviour and language drastically changed, he told her she became a 'clone of Regina'. Regina went to the bedroom, saw them together and at that moment - Cady vomited on Aaron.
Aaron left the party, while Cady started chasing him. She was confronted by Janis and Damien who just returned from the art show, after they won the first prize. After having an argument, Janis threw the picture she draw, causing Cady to realize what she's done.