The X Factor-winning group, who recently won the PopJustice Twenty Quid Music Prize for the third time, had to take to the stage without Perrie Edwards – who is rumoured to have a gastric illness.
They were at the iHeartRadio music festival in the US and soldiered on as a three-piece, although their performance was marred by poor sound quality.
Fans blasted organisers for seemingly scuppering the performance, which comes as the girls continue to fight for a higher profile in the US.
Speaking backstage, Jesy Nelson explained: “She’s not feeling very well. So yeah, she’s resting.”
A spokesperson told the Mail Online: “Perrie was taken ill overnight and had to go to hospital with gastric problem.
“She saw a doctor and was sadly unable to perform.”
However it was the sound issues that proved a bigger point of contention amongst fans.
One tweeted: “Well we couldn’t even hear them because of the damn microphones! What was up with the sound??”
“HOW WILL YOU EXPLAIN THIS I HEART MESS RADIO?” raged another.
Little Mix are still as strong as ever after netting a string of consecutive chart hits, and their first No1 album with most recent effort Glory Days.
They’re set to resume their Glory Days Tour with a series of UK arenas later this year.