The "We Can't Stop" singer, who was hospitalized April 15 after suffering a severe allergic reaction to antibiotics, is battling a recurrence of the same symptoms and has been forced to postpone the start of her European tour.
"After a period of improvement, Cyrus' symptoms have returned and doctors will not permit her to travel," her management confirms to PEOPLE.
The 21-year-old, who was told that her allergic reaction could last for up to 27 days and remains under a doctor's care, previously postponed the remainder of her U.S. tour this month and has now rescheduled a concert in Amsterdam on Friday, and one in Antwerp, Belgium, on Sunday. She's expected to be well enough to perform in London on May 6.
While she hasn't commented publicly on this latest setback, Cyrus Tweeted a message on Tuesday filled with Sad Kitty emoticons.
Cyrus told Ryan Seacrest in a radio interview Monday that she had ended up in hospital after being prescribed medication for the flu while she was on the road with her Bangerz tour.
"It was so scary," she said. "I had basically been poisoning myself with something I didn't know I was really scary allergic to."