It’s still a mystery to me why anyone would spend their rent money on a ticket to watch a goat sing.

While performing her hit single “What’s My Name,” in Lisbon over the weekend, 23-year-old Rihanna forgot the lyrics to her own song, and slurred some of her words. At about the 1:32 mark, Rihanna gestured to someone in the audience indicating that she was about to hurl.

Moments later Rihanna disappeared into the darkness off the side of the stage. Rumors began to swirl almost immediately on Twitter that the singer was stumbling drunk and vomiting during her concert. Later that night, Rihanna took to her Twitter page and admitted that she did indeed throw up the contents of her stomach offstage, which consisted of a half eaten hot dog and about a quart of alcohol.

We broke the story months ago that Rihanna’s record label was concerned about Rihanna’s drinking and rumored substance abuse problems. Sources say Rihanna has been on and off the wagon in the past month as she deals with her personal issues, including chasing after a man who is already in a long term relationship.

Sources also say Rihanna is deeply depressed over the poor sales of her new album, Talk That Talk, which just barely cracked the 300K sales mark in the U.S. last week. With over 10 million followers on Twitter.com, she believes she should have sold a lot more.

It’s not your music, it’s your stank attitude, Rihanna.