After performing in Denver, Colorado, Lady Gaga spent some time in the hospital for altitude sickness earlier this week.

The 28-year-old pop icon, who is currently traveling on the artRAVE: The ARTPOP Ball world tour, performed at the Pepsi Center in the Mile High City on Wednesday, reports ABC 7 News Denver. However, after the show, she shared a selfie of herself on Instagram lying in a hospital bed while wearing an oxygen mask. 

"Altitude Sickness is no Joke! #hitThatHospitalS*** #artRaveDenver many true ravers crowd tonight," wrote the singer, who is affectionately known as Mother Monster by her fans.

According to WebMD, altitude sickness can occur when people reach an altitude of 8000 feet or more and do not get enough oxygen from the air, which becomes thinner at high elevations. As a result, the illness can cause headaches, nausea, loss of appetite, vomiting, dizziness, weakness and interrupted sleep.

It's a pretty common occurrence in Colorado, which is why the Denver.org gives visitors advice on how to cope with the altitude on, notes People magazine. Tip number four on the website advises people to "Watch your physical activity."

Altitude sickness is also common in other mountainous regions and high-altitude countries like Peru, Ecuador and Nepal.

Although mild altitude sickness is common, the illness can become serious and affect a person's lungs and brain.

"When this happens, symptoms include being confused, not being able to walk straight (ataxia), feeling faint, and having blue or gray lips or fingernails. When you breathe, you may hear a sound like a paper bag being crumpled. These symptoms mean the condition is severe. It may be deadly," states WebMD.

The "Applause" singer is scheduled to perform next in Seattle, Washington, on Friday as she wraps up the North American leg of her tour. 

She will then head overseas to Asia, Australia, the Middle East and Europe before closing out artRAVE in Paris on Nov. 24.