EXCLUSIVE: How Ariana Grande is Stepping Back From Spotlight Amid Health Concerns

Ariana Grande stunned fans by announcing she's "taking a step back" after her current tour ends on Sept. 1, claiming in a statement she was "ending it on a high note, both healthily and happily," RadarOnline.com can reveal.

But sources said the 33-year-old Wicked star – who's long been criticized for her stick-thin appearance – is putting on a brave face.

In addition to putting concerts on hold, the Grammy winner has also pumped the brakes on an upcoming theater stint in London, due to start next summer.

The move comes after intense public scrutiny over the former Nickelodeon star's plummeting weight. One top doc previously estimated to RadarOnline.com that the 5-foot-2 star likely weighs a terrifying 87 pounds.

"At that weight, she would be severely calorie-deficient," warned Dr. Gabe Mirkin, who doesn't treat Grande. "That puts her at high risk for serious problems."

As readers know, the scrawny songbird has had a string of other setbacks. In January, sources said she was devastated after Oscar voters snubbed her for her starring role in Wicked: For Good.

"She's spiraling into a terrible funk," warned an insider at the time, adding, "Ariana feels like the rug has been pulled out from under her."

Then earlier this year she split from her boyfriend of nearly three years, Wicked costar Ethan Slater, 34, before reportedly reuniting with former beau Ricky Alvarez, 35.

"There's been a lot of upheaval in her life lately," said the source. "The break will do her well."