gagasicklogo230.jpgLady Gaga has been hospitalized six times for yo-yo dieting ... and that was just last year alone, says her ex-tour manager, David Ciemny.

Ciemny claims the Poker Face singer is "sick" and "obsessed with weight" because of the extreme pressure to be thin. He admits that she once lost 20lbs between a first and final costume fitting.

"When I say she was sick, I mean physically and mentally," adds Ciemny.

In a new unofficial biography titled, Poker Face: The Rise and Rise of Lady Gaga, author Maureen Callahan supports Ciemny's claims by writing, "The pressure on her to lose weight was very high."

Gaga, 24, cancelled four shows in the U.S back in January after collapsing on stage reportedly due to exhaustion and dehydration. Her illnesses didn't stop there. This past May, she revealed that she tested "borderline positive" for lupus.

In addition to her struggles with weight, the ex-tour manager also claims the singer fired 150 staff members between 2008 and 2009.

Lady Gaga's rep has not returned our calls for comment.