Britney Spears' former backup dancer is speaking out about the bombshell accusation from Kevin Federline. In an excerpt of his upcoming book You Thought You Knew, which drops on Tuesday, the dancer claimed he caught the pop star cheating on him with a woman! Whoa! It happened early on in their relationship, right after a night out at the club in Amsterdam during The Onyx Hotel Tour in 2004. Britney, K-Fed, and her dancers and staff returned to the hotel, and she disappeared later, saying she would be right back.