Lady Gaga posted a selfie on Thursday, Aug. 7 from the hospital alerting fans that she was suffering from altitude sickness. According to the New York Daily News, the 28-year-old "Artpop" singer had just wrapped a concert in Denver when she fell ill and was taken to the hospital.

"Altitude Sickness is no Joke!" Gaga captioned a photo of herself wearing an oxygen mask. "#hitThatHospitalS**t #artRaveDenver many true ravers crowd tonight."

It's not clear if Gaga has been released from the hospital yet but her next artRave tour stop in Seattle is still scheduled for Friday. So what is altitude sickness? According to WebMD, it "occurs when you cannot get enough oxygen from the air at high altitudes. This causes symptoms such as a headache, loss of appetite, and trouble sleeping."

It goes on to say that altitude sickness can be dangerous and it's "smart to take special care" if you will be doing activities that requires you to go to high-altitudes. The singer is the latest celebrity to get sick during a tour.

Recently Paul McCartney took a break from his tour because of a virus and Miley Cyrus had to reschedule several dates due to a severe allergic reaction to medication she was taking.

"It was so scary," Cyrus previously told Ryan Seacrest about her hospital stay. "When you're allergic to something you're basically poisoning yourself. I had basically been poisoning myself with something I didn't know I was really scary allergic to."

She explained that she had come down with a cough while on tour and was prescribed antibiotics to treat it but found out later that she was allergic to the meds.

"Every two or three hours, I would be ok and then all of a sudden it would happen to me again," Cyrus said, referring to her allergic reactions. "It was insane. I was begging the doctors, 'Let me go out to go the show!' it was basically no way."