Pop star Pink has had her gallbladder removed in hospital, according to U.S. reports.

The Trouble singer tweeted fans a photo of herself in a medical centre in Los Angeles after admitting herself last week, but she refused to state why she was there.

It was clearly not a minor illness because the singer had to pull out of a performance at a fundraising gala for U.S. President Barack Obama in Los Angeles on Wednesday night.

And now sources tell Life & Style magazine, Pink has had her gallbladder removed.

One insider says, "She had these bad stomach pains and didn't know what it was, and then it got really bad. They realized she needed to have her gallbladder taken out."

Pink, who celebrated baby Willow's first birthday from her hospital bed over the weekend, was replaced by Glee star Darren Criss at the fundraiser.