The 21-year-old singer was forced to cancel her Bangerz tour appearance at Kansas City's Sprint Center on Tuesday.
Her current condition is not clear, but she has tweeted fans from her hospital bed.
"Kansas I promise I'm as heartbroken as you are," Miley wrote. "I wanted so badly 2 be there 2night. Not being with y'all makes me feel s****ier that I already do.
"Mr Octopussy & some amazing Drs are taking good care of me," she added, next to a selfie from her bed.
Miley has been feeling under the weather for a while now.
On Monday 14 April, she tweeted, "I woke up like disssss #SICK," followed by a message several hours later that read, "Fever fever go away".
The singer also cancelled her tour stop in Charlotte, North Carolina just over a week ago after she was "diagnosed with the flu" and doctors "ordered her to rest".
On top of that, Miley has been dealing with the death of her beloved dog Floyd, who was killed by a coyote while she was away on tour.
"I don't wanna say it because I don't want it to be real… But my precious baby Floyd has passed away. I am broken," she tweeted on 1 April.