Laura Trott was seen being sick at the side of the track at the Rio velodrome on Monday night due to problems regulating the acid in her stomach.
Medics rushed to the side of the Team GB cyclist as she vomited into a blue plastic bag.
Trott is sick after most races due to the issue.
She became the first GB woman to win three gold medals earlier this week - and looks ideally placed to win a fourth in the omnium.
On Monday she won two of the first three disciplines of the omnium, coming second in the other.
She came second in the opening scratch race and won the individual pursuit and elimination race – a discipline which has become her trademark – to lead on 118 points.
Belgium’s Jolien D’hoore was second on 110pts and Sarah Hammer of the United States third on 108.
British Cycling head coach Iain Dyer said: “Once again, confidence has got to be sky high.
“Who wouldn’t want to go into day two of an omnium with a lead like that?
“She’s queen of the elimination once again. It’s an event she loves and everyone else ducks for cover and needs a lie down in a dark room afterwards.
“She’s clearly in super form, it sets her up well for day two.”
Laura Trott will be competing in the women’s omnium starting with the time trial at 2.57pm and concluding with the points race at 9.05pm.
Meanwhile, Trott’s fiancé Jason Kenny is also hoping for glory on Tuesday evening in the men’s keirin.
The pair’s relationship came to light at the London 2012 games when, their competition complete and two golds each won, they were pictured kissing at the beach volleyball at Horse Guards Parade, sitting a row behind David Beckham.
They earned themselves the nickname “the golden couple” thanks to their medal hauls.