The Teenage Dream singer has been feeling sick over the past week and she told fans how she was determined to perform in Kentucky and Louisiana.
Katy, 26, had hoped to get better for her performance in Grand Rapids, Michigan on the 10th anniversary of the 9/11 terror attacks in the US - but she ended up in bed after being diagnosed with a respiratory infection.
She wrote on her Twitter page: 'Sick in bed and watching the memorials today. Taking time to remember all the lost & all the heroes. GodBlessAmerica.'
The Grand Rapids concert has been rescheduled to December 1.