An insider says Mariah Carey is two months pregnant -- and she's over the moon

Vacationing in Cabo San Lucas, Mexico, with her new husband, Nick Cannon, Mariah Carey turned heads -- but not for the usual reasons.

Known for sunning herself in barely-there bikinis, Mariah was clad in a baggy cover-up that masked her midsection. "It was weird that she dressed so conservatively," says an onlooker on the beach November 15. "She kept her hand on her belly a lot."

A day before she left for Cabo, Mariah, 39, had people talking at the opening of Miami's newly renovated Fontainebleau hotel -- again, because she wasn't acting like herself. "She said no to a champagne toast backstage," says an insider. "She toasted with a water bottle instead."

According to an insider, Mariah's un-Mariahlike behavior stems from one very happy reason: She and Nick are expecting a baby. "Mariah has always wanted a baby," says a close friend of the singer. "She knows she is getting older, and to her, a baby is a beautiful cherished gift."

Mariah's dream may have come true -- a source says she is eight weeks along -- but her pregnancy wasn't planned.

"Mariah had been on birth control pills. She got sick, and she was on antibiotics, and that made her birth control less effective," the source explains. "That's how she got pregnant. It was a surprise."

A child of divorced parents, Mariah once said she wanted to wait to have a baby until she was in a stable relationship with "someone who would be a great father." And it appears that she's found exactly that with Nick, 28. "Nick is such a great man," Mariah's friend says. Mariah couldn't agree more. "I think we would make good parents," she says, "and that we'd be able to figure everything out together."