Trouble at Shady Pines?

PAGESIX is reporting about a possible pop rivalry at the retirement home...

Is there a beef between Madonna and Janet Jackson? Madge was chummy with fellow singer Shakira at Butter Monday night (our spy said they showed up "hand in hand") and was spotted dancing on banquettes with Demi Moore, Ashton Kutcher and Penelope Cruz. But Jackson was holed up a few booths down and, "She was not invited to join Madonna's crew," said an onlooker, who told us the table drank "seven bottles of champagne and a ton of beer."

Is this Jessica Simpson and Britney Spears in a few years?