Britney Spears has already left rehab - the morning after checking into Promises in Malibu, according to Entertainment Tonight. "It was against their advice," a source confirmed to People magazine. "If she comes back for treatment, Promises will have to assess the situation. Sometimes people come back, but it doesn't look good." The rehab facility, which has treated high-profile stars such as Ben Affleck, Robert Downey Jr., Matthew Perry and Charlie Sheen, is a voluntary program - which means patients are able to leave if they wish.Spears's recent erratic behavior is raising more than eyebrows for people concerned about the popstar's two young sons. TMZ is reporting that the LA County Department of Children and Family Services has received numerous calls to their hotline about Spears, and a former nanny spoke to Us mag about her experiences working for the young mom, saying that Spears "gets overwhelmed" when she alone with her boys. "She gets so frustrated when they cry, asking, 'How do I make it stop?'" The celebrity child minder said of being hired for the job, "Britney didn't ask me one thing about my child-care experience. She only wanted to know about my personal life... The agency that called me emphasized that Britney was looking for a nanny who was young and hip because they wanted her to interact with people her own age. Basically, Britney wanted a friend." And of working for the popstar, the source said, "Britney asked me and one of her nannies to come to her room to watch her try on outfits for a party one night - then she stripped down naked in front of us!... One nanny told me that Britney will hold her kids for 10 minutes and then say, 'I'm done now. You can take them.'... She doesn't like it when Sean prefers the nanny, so she fires them and looks for a new one."