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Britney Spears and instability go together like Kevin Federline and manpris, but is she experiencing a nightly sense of unsteadiness while on her Circus tour? So claims the London Daily Mirror, which says a particular onstage dance move is making her stomach flip-flop.

"The choreography is insane -- it's so fast it is really taking its toll on her," a source tells the paper. "But there is one move during 'Touch of My Hand,' where two of the guys dangle from the ceiling and pick her up upside-down, which is really getting to her."

Seems the ensuing spinning from that position is making the blood rush to Brit's noggin, and "when she gets down she complains of nausea and dizziness," continues the mole. "There have been a couple of times when she ran off stage and vomited. It's not a pretty sight."

Neither is the detailed way the insider describes her supposed discombobulated state: "It's more sickening than downing 20 shots of peach Schnapps and then spinning on your head. She has asked if they can possibly change it, still keeping it dramatic -- just minus the projectile vomit." (And minus the hair loss. While performing the song the other night, several of her extensions were left behind when the dancer hoisted her into the air.)

Perhaps Britney's alleged queasiness is behind recent blood-chilling rumblings that Spears may be pregnant, which surfaced this week courtesy of Ryan Seacrest, who said on his radio show he'd heard about it from a "reliable source."

But Brit's rep insists Sean Preston, 3, and Jayden James, 2, aren't getting a new sibling anytime soon, telling Perez Hilton, "She's absolutely not pregnant. I have no idea where this ridiculous rumor came from. Ryan has communication with our camp and could have checked this before saying it."

This isn't the only baby rumor floating around, however. This week's Star has a doozy about how Spears and Federline have supposedly talked about having more kids together. Go ahead and shudder -- we'll wait.

K-Fed must have squeezed in that talk between issuing a denial to In Touch over its claim that he -- and his meatier frame -- have been tapped by NutriSystem to be a spokesman for its Mens Plan program.

Kevin wants to lose weight, and he needs to make money, a pal tells the tab. Hes seriously thinking it over.

But Federline's rep rejects the weight loss shilling report as untrue.

As for Britney, she was feeling in the pink enough to give her green thumb a workout on Tuesday. Her official Web site posted a picture of the flowers she planted with Sean and Jayden's nanny in the courtyard of her palatial new Calabasas, Calif., estate.

Next: 'Twilight' rumor mill keeps cranking