Now People reports, an ultimatum from Kevin Federline, is the reason Britney Spears is back in rehab .
Upset by the news that Spears had checked out of Promises in Malibu on Wednesday, Federline and lawyer Mark Vincent Kaplan scheduled an emergency hearing to seek physical and legal custody of their sons Sean Preston, 18 months, and Jayden James, 6 months, says a source.

Federline, 28, gave his estranged wife a choice: Either she got back into Promises that night or Federline would go to court in the morning, says the source.

Last Friday Federline spent an hour visiting his ex at Promises; the next evening he also brought the boys, and they were joined by Lynne.

"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."

A typical stay at Promises lasts 45 days – and Spears "is getting no special treatment," says her manager Larry Rudolph. "She has a regular room, a regular routine. She is doing exceptionally well."

Before checking in to Promises rehab center in Malibu for the second time, Britney Spears enjoyed a strange, day with two new friends reports US weekly.

On February 17, a newly bald Spears arrived at L.A.'s chic Mondrian hotel. But the star was denied a room due to lack of credit cards or cash.

By 11 a.m., Spears, 25, had stripped down to a bra and panties poolside, then shaved her legs in the pool bathroom.

“It was sad,�? says a source. “It looked like she really needed a friend.�?

Around noon, Spears began chatting up a woman in the bathroom, who offered to loan the pop star a bathing suit.

Spears followed her new friend to her hotel room where -- after changing into a borrowed bikini -- she raided the minibar.

“She grabbed four or five bottles and just started mixing everything and drinking them.�?