So why did Britney Spears check into rehab only to leave twice, then finally check her self back in last week? Turns out that she got an ultimatum from Kevin Federline.

Federline was upset by the news that Spears had checked out of Promises in Malibu last Wednesday, so he and his lawyer mark Vincent Kaplan scheduled an emergency hearing to seek physical and legal custody of his and Britney’s sons Sean Preston and Jayden James.

It’s said that Federline, 28, gave Britney a choice – either go back into Promises that night or Federline would go to court the following morning.

That evening, at about 7 pm, Spears showed up outside of Federlines home in Tarzana, California. She and her friend were follwowed by 20 paparazzi. Federline didn’t appear so she left.

A photographer followed her to a gas station. Britney was enraged that she’d been followed and she grabbed an umbrella and started to whack his SUV. Later that night Spears mother, Lynne, took Britney to Promises. A friend of Federlines said “Britney scared everyone real bad.”

Last Friday Federline spent an hour visiting Britney at promises, the next evening her brought the boys to visit. Spears mother joined the group that day as well.

“Kevin is working closely with Lynne to let Britney know how serious it is that she gets help,” says one family friend. A source close to Federline notes that “Kevin loves Britney … nothing has changed about that.”

Right now the boys are “being taken care of by Kevin,” says a source. “Lynne is helping.” Eventually, says a Spears family friend, Federline, who will turn 29 this month, “would like Britneyto have the kids because he thinks she is a good mother.”

A typical stay at Promises is 45 days. Will Britney make it that long considering it’s her third try in a week? Spears’ Manager Larry Rudolph says that she’s not getting any special treatment at the rehab facility. “She has a regular room, a regular routine. She is doing exceptionally well.”