The star attended the Pier 1 Christmas Pop Up Storybook launch in SoHo on December 1, reported People magazine.
"She had fallen ill earlier in the day, but participated in the event last night as much as she could," a source said,
A rep for the 45-year-old singer said she "had been feeling under the weather with the flu over the past few days and was fighting it to the best of her abilities.
"She is doing well now and will be performing as scheduled this weekend. She thanks her lambs for the continued support and well wishes.