THE ENQUIRER exclusively reports Britney Spears was furious when she discovered her ex K-Fed out partying with his ex!

K-Fed and former GF Shar Jackson – mom of his two oldest kids – hit the Madden 09 video game launch party while Brit was stuck at home, caring for 2-year-old son Sean‘s ear infection.

Kevin, who has custody of his and Britney’s two boys, agreed to send the boys for an overnight visitation after she promised she could take good care of the boys, give Sean his medication on time and take his temperature, a source told The ENQUIRER.

Although Britney knew Kevin planned to go to the  vid game kickoff, when she discovered he was with Shar, she went ballistic!

Brit called K-Fed repeatedly while at the party, leaving him panicky messages.  She told him that she didn’t remember what meds Sean had to take and that he was whining constantly for dada.
"Britney was even asking how to work the thermometer to take his temperature," the source revealed.

Kevin first called the bodyguard who accompanies the boys when they go to Britney’s, and then he called Britney’s father, Jamie, to check on Sean.  Both Jamie and the bodyguard assured Kevin his son was fine.

"When Kevin wouldn’t take her calls, Britney started text-messaging him and kept texting him for much of the night," said the source.

What Britney finds most infuriating is that Kevin lets Shar spend "her money" – the court-ordered $20,000 Brit shells out to Kevin each month in child support.

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