"I don't cancel shows. Like, I will drag myself onto the stage and perform if I have the flu or whatever, but in the U.K. we had to cancel a couple of shows," Underwood told Fallon. "Basically I got a viral thing and I woke up and I had vertigo. I could not stand up. It was crazy."
All seemed well for the "Love Wins" singer following a set at the Tuckerville Festival in The Netherlands on Sept. 1. She took to Instagram to send her thanks to the fans after her set and jetted off to Berlin, Germany.
"Had so much fun tonight at the @TuckervilleNL festival! Thank you, fans, for singing along with us and making me feel so welcome," she wrote alongside a photo collage from her time at the event. "And thank you @ilsedelangemusic for having me up on stage with you! I already can’t wait to come back to Holland! Now, on to Berlin! ❤️"
It was in Germany, though, where Underwood fell ill.
"I ended up in a German hospital for three days," she told Fallon. "They were so sweet and so nice and we were like in our room trying to learn German and communicate with the people. I didn't learn anything."
The mysterious sickness caused Underwood to cancel at least two shows in the U.K. On Sept. 5, The Long Road Festival, which took place in Leicestershire, United Kingdom, announced that Underwood had canceled her appearance "due to illness."
"We’ve just heard that Carrie is unable to play this weekend due to illness, and our primary concern is her speedy recovery. We know you’ll have questions, and we promise to come back to you as soon as we can," the festival, which took place on Sept. 8, wrote on Twitter alongside a photo of a lengthy statement promising "a fantastic line up of music and experience" to its attendees.
We’ve just heard that Carrie is unable to play this weekend due to illness, and our primary concern is her speedy recovery. We know you’ll have questions, and we promise to come back to you as soon as we can. pic.twitter.com/4GS3NfnlDY
In a second tweet the following day, the festival called Underwood "irreplaceable" before announcing that Aaron Watson would headline the show originally slated for Underwood and that Danielle Bradbery would be extending her set.
Underwood was also supposed to appear at BBC's Radio 2 Live in Hyde Park, U.K. on Sept. 9, but had to cancel that show too.
"Unfortunately Carrie Underwood is unable to join us at Radio 2 Live in Hyde Park due to illness," the news outlet wrote on their website. "We wish her well and look forward to welcoming her to Radio 2 in the future."
The illness comes less than a year after Underwood's serious fall -- where the "Before He Cheats" singer broke her wrist and had to get 40 to 50 stitches on her face -- last November.