Troubled star Britney Spears has accused Kevin Federline of betraying her - in the lyrics of her new song. The 25-year-old singer reveals the heartbreak behind her two year marriage to Federline, 29, which ended in divorce in July this year (07). In the track Why Should I Be Sad - which has been co-written by singer/producer Pharrell Williams - Spears makes references to her ex's love of cars, money and other women. In one line she sings, "It just seemed that Vegas/ Only brought the playa out of you." A source tells, "The song is definitely about Britney and Kevin's relationship when they were married. Britney entertains the idea that she stayed home taking care of the kids while Kevin was out partying."