Marilyn Manson -- Don't Worry, It's Just the Flu

Marilyn Manson isn't dead ... he was just suffering from a bad case of the rumble guts last night when he passed out on stage in Canada after coming down with the flu ... TMZ has learned.

Sources close to the rocker tell TMZ ... Manson had been feeling like crap all day long (from various flu symptoms) ... but didn't wanna call off his concert in Canada.

Problem is, when MM hit the stage, he started feeling worse -- vomiting on stage several times before collapsing in the middle of his hit "Beautiful People."

Sources say Manson was NOT taken to the hospital -- instead, he went back to his hotel room and got some rest. We're told he still feels "drained" -- but he's focusing on recovery and is determined to perform tomorrow night in Calgary.