K-Fed partying with ex-girlfriend Shar Jackson makes Britney Spears a very unhappy girl. Britney has been left livid after finding out that her ex-husband, Kevin Federline, is once again bosom buddies with the mother of his two oldest children.

“We were friends before we were a couple and we’re always going to be friends,” Shar told People as she welcomed Kevin to her 32nd birthday party Blush Nightclub Las Vegas on Friday night. “We have two amazing kids together. Kevin is my boy.”

The weekend b-day bash wasn’t the exes first public outing. Britney cared for son Sean Preston, who was ill with an ear infection, while Kevin and Shar attended the Madden NFL 09 Video Game Launch Party on August 7.

“When Kevin wouldn’t take her calls, Britney started text-messaging him and kept texting hime for much of the night. Britney also called her mother Lynne and sister Jamie Lynn, telling them that Kevin and Shar were back together and wondered how long this had been going on behind her back,” an insider tells the National Enquirer.

Brit is also angry that Kevin spends a large portion of her monthly $20,000 child support agreement on Shar.

“Shar has a credit card that belongs to Kevin and it’s as if she could charge whatever she wants for herself or the kids, no questions asked. Kevin also pays her rent and other bills.”

Shar is even helping Kev potty-train Sean Preston, who turn 3 in September.

“Shar said Kevin will always go back to her after he has had his fun. She knows she’ll always by Kevin’s first true love.”