Vonn's spokesman, Lewis Kay, wrote in an email that the four-time overall World Cup champion was "awaiting results from diagnostic testing for severe intestinal pain."
Vonn's ski technician, Heinz Haemmerle, told The Associated Press that this isn't the first time the racer has gone to the hospital in Vail.
Haemmerle, who prepares Vonn's Head skis and has been on hand in Colorado awaiting her recovery, said that Vonn hasn't trained since going out in the second run of the season-opening giant slalom on Oct. 27 in Soelden, Austria.
"She told me she feels bad and has pain all over her body and that her bones are hurting ... she's been (to the hospital) two or three times. This is the first time she's stayed overnight," Haemmerle said in a telephone interview. "The coaches also don't know. First they told me we would train again Monday, then Wednesday, now the end of the week."
Vonn recently requested to compete in a men's downhill race, only to be rejected by the International Ski Federation. She was hoping to enter the men's race Nov. 24 at Lake Louise, Alberta. Had she been allowed to compete against the men, Vonn would have missed the two women's races in Aspen, Colorado, because they take place the same weekend.
Vonn also skipped a slalom in Levi, Finland, last weekend, although Haemmerle said that was their plan to gain more training for Aspen even before the illness.
Haemmerle said Vonn attended the Snow Ball in New York after Soelden, then went to Colorado.
"There's been no news, so we're kind of worried," Head racing director Rainer Salzgeber said from Austria. "It's not good."
Both Haemmerle and Salzgeber confirmed that Vonn has no broken bones or skiing injuries.
While extremely successful, Vonn's career has been marked by a series of ski-related injuries — most notably at her last five major championships.
She had to withdraw midway through the 2011 world championships in Garmisch-Partenkirchen, Germany, with a mild concussion and battled through a severely bruised shin to win the downhill and take bronze in the super-G at the 2010 Vancouver Olympics.
At the 2009 worlds in Val d'Isere, France, she sliced her thumb open on a champagne bottle after sweeping gold in downhill and super-G, forcing her out of the giant slalom.
At the 2007 worlds in Are, she injured her knee in training and missed the slalom and giant slalom; and at the 2006 Turin Olympics, she had a horrific crash in downhill training and went directly from her hospital room to the mountain to compete in four of her five events.
About a year ago, Vonn separated from her husband of four years, Thomas Vonn, and went on to her most dominant season with 14 wins and reclaimed the overall title from Maria Hoefl-Riesch by setting a new overall points record at 1,980, 578 ahead of her nearest competitor.
Vonn's next races are the giant slalom and slalom in Aspen on Thanksgiving weekend, followed by the women's events in Lake Louise, her most successful stop on the circuit. The highlight of this season will be the worlds in February in Schladming, Austria.
AP Sports Writer Pat Graham in Denver, Colorado, contributed to this report.