The 27-year-old singer, who had two operations last week for an undisclosed condition, didn't miss her set at New Look Wireless Festival in London's Finsbury Park, where she confessed she was still feeling terrified about her illness, reported Daily Mirror.
"Some of you may know and some of you may not know, I came out of hospital two days ago after two operations last week and the fear is unbelievable.
"I didn't think I was gonna make it today and I have been told not to be here, but ain't nobody got time for that. I can't miss shows in London," she said.
Following a performance of "Sweet Talker", she told crowds: "You know what? I swear anaesthetic makes your body lose all sense of rhythm."
The performance comes just a week after Jessie - who cancelled a string of Australian tour dates in March due to her illness - confessed she was "in a lot of pain" while working with doctors.