Credit: Ray Mickshaw/FOX

Are things already getting toxic for Britney Spears and The X-Factor?

On Thursday, to kick off a reported $15 million gig, the pop superstar, 30, had her first day at the judges' table next to Simon Cowell, L.A. Reid and fellow newcomer Demi Lovato in Austin, Texas.

TMZ claimed that Spears angrily walked off the set after one hopeful butchered a rendition of her hit "Hold It Against Me."

The engaged singer herself quickly took to Twitter to slam the report. "Britneywalksoff??? LOL was just taking a little break people," Spears wrote. "I am having the BEST time!!!"

According to a set source, Spears (looking sexy in a florescent, tight, short pink dress with sparkly stilettos) did indeed abruptly leave during the "Hold It Against Me" audition -- but she didn't appear "unhappy or upset."

Still, as fellow Disney alum Lovato bantered gamely with Cowell throughout the audition, Spears seemed slightly "uptight," a witness says. "Her body language was reserved."

By contrat, "Demi, Simon and LA seemed very relaxed and engaged with contestants and audience," according to the witness, giving "pointers and honest critiques" to contestants.