About an hour after she helped do the midnight countdown in the Pure nightclub inside Caesars Palace, Spears was talking to a male dancer when she suddenly slumped to the floor, the Las Vegas Review-Journal reported.
Witnesses said she was wrapped in a blanket and all but carried out of the club, the New York Daily News reported. However, other witnesses told People magazine "She was absolutely not carried out of the club."
Her manager, Larry Rudolph, downplayed the incident Monday, saying she had been drinking champagne, but not that much.
"She had been traveling all day. She got real tired. It was a late night for her," Rudolph said.
Since filing for divorce from Kevin Federline in November, Spears has made headlines for partying heavily several times, including being pulled from the stage of a Hollywood burlesque club.