"The papers have apparently blown things out of proportion," Amy Nelson told the Los Angeles Times Monday.
"He did cancel the [Colorado] show because he did not feel that he could give an adequate vocal performance in the high altitude," she said. "He had gone to the hospital a couple days before to treat an injury on his leg from a bicycle accident. Perhaps that's where the hospital reference got mixed in. Or maybe people just panicked, as they usually do when he cancels a show."
The event was to have been in Castle Pines Village, about 20 miles south of Denver. The unincorporated community lies 6,000 to 6,500 feet above sea level along the Front Range of the Rocky Mountains.
The Denver Post earlier quoted fundraiser host Fred Bartlit as saying, "I was told Willie woke up and had trouble breathing, so he was taken to the hospital."
The event, a fundraiser called Lulu's Barkin' BBQ for the Dumb Friends League, was held at Bartlit's home.
The Denver-based Dumb Friends League is an animal shelter and humane society.
Nelson, 79, "feels much better, and no other tour dates are being postponed," Amy Nelson told the Times.
His next scheduled performance is Tuesday night at the House of Blues in Dallas.
Nelson is featured among several country and bluegrass artists whose music is heard in the forthcoming Prohibition-era gangster film "Lawless."
Nelson sings "Midnight Run," about running bootleg liquor, the Times said.
Nelson, who has actively performed since 1956, created hit albums like "Honeysuckle Rose" and hit songs including "On the Road Again," "To All the Girls I've Loved Before" and "Pancho & Lefty." From 1985 to 1995, he was part of The Highwaymen, with fellow country singers Johnny Cash, Waylon Jennings and Kris Kristofferson.