06/04/2007 at 04:00 PM EDT
had a whirlwind getaway in Mexico this weekend, where she let her new hair down – and offered some friendly advice to fellow tourists.
Over her three-day vacation, the singer, 25, stayed at the Four Seasons in Punta Mita, 26 miles from Puerto Vallarta.
While strolling on the beach with a pal at the Four Seasons, Spears stopped an older couple, also guests at the hotel, to warn them: "Be careful, there are lots of man o' war jellyfish washing up on shore, you don't want to get stung," a source tells PEOPLE.
According to the source, Spears was "very sweet" – and the couple didn't recognize her until she started talking to them.
"She seemed happy and carefree with her friend," says the source.
Spears, who went to Mexico on Thursday and returned to Los Angeles on Sunday, was photographed over the weekend in Mexico wearing a turquoise-and-white sarong with her blonde hair extensions in a
The weekend away comes just days after she posted a message on her Web site
saying she was at "rock bottom" when she went to rehab earlier this year and
thanking fans for their support. "I love you for still loving me," she wrote.
Spears, who has been recording a new album, also recently performed several shows in Los Angeles, Miami and Las Vegas in recent weeks.
Reporting by JENNIFER GARCIA