The "Applause" singer took to Instagram early Thursday morning to share a selfie from the hospital wearing an oxygen mask.
"Altitude Sickness is no Joke! many true ravers crowd tonight," she captioned the photo along with the hashtags #hitThatHospitalShit and #artRaveDenver."
Gaga's hospitalization might've been prompted by the intensity level of her performance as Denver's government website encourages visitors to "watch [their] physical activity" while visiting the city.
The singer's next show in Seattle on Friday is expected to go on as planned.