NEW YORK —  Marie Osmond says she doesn't know why she fainted during ABC's live broadcast of "Dancing With the Stars," but it may have something to do with allergies.

Osmond and her partner, dancer Jonathan Roberts, the first performers on Monday night's show, had just finished a samba and were listening to comments from the judges when she suddenly toppled.

"She was laughing and then sank like a stone. ... Everyone froze," said ABC publicist Amy Astley. As the show cut to a commercial, Osmond started to rise and was helped by others, Astley said.

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"I feel bad that it happened," the effervescent 48-year-old singer told "Entertainment Tonight" in an interview that was to air Tuesday night.

"Right at the beginning ... I started getting lightheaded. The only thing I can think of, I have allergies. ... The air quality (in Los Angeles) is terrible. And I think I just couldn't get my breath," Osmond told "E.T."

The hit series, which pairs celebrities with professional dancers, puts its stars through rigorous dance routines.

She explained on-air that "once in a while that happens to me when I get winded. I stop breathing." She punctuated her remark with a giggle.