While the problems between Britney Spears and Kevin Federline play out in the courts, it appears the pop star plans to expose – very publicly – some of the frustration she felt in her former marriage.

A source tells PEOPLE that "one of the most personal songs" in Spears's upcoming album Blackout, to be released Oct. 30, was written by singer-producer Pharrell Williams – and seems to make clear her feelings of betrayal.

Called "Why Should I Be Sad," the track makes references to money, cars – and other women. As Spears, 25, sings in one part of it: "It just seemed that Vegas/ Only brought the playa out of you."

Speaking to PEOPLE at the Hennessey Artistry concert in Los Angeles, Williams, 34, said of Spears: "At the end of the day she is a sweet person. She's just made a couple of decisions that none of us would necessarily agree with, but you try being that age and living under that kind of pressure. I wish her well. I pray for her. I hope she fares well throughout all of this craziness."

Says a source about "Why Should I Be Sad": "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. The track ... will be very revealing for her fans."