7:33 AM PT -- 2/7 -- A rep for Heather tells us she suffered a skull fracture as a result of the fall, and continues to undergo tests at the hospital to figure out what happened. We're told she had not consumed any alcohol prior to the show ... and tested negative for COVID.
Comedian Heather McDonald fell hard during a show in Tempe, Arizona, but it was no joke ... it was a medical emergency.
Eyewitnesses tell TMZ ... the former "Chelsea Lately" star was headlining the sold-out show, when she delivered her second joke ... "I'm vaxxed, double vaxxed, boosted ... and flu shot and shingle shot and haven't gotten COVID and Jesus loves me most."
As soon as she delivered the joke, she just collapsed and hit her head on the floor. Everyone thought at first it was all part of the act, but it was not. She was conscious, but clearly, all was not well. An EMT and a nurse happened to be in the audience ... they rushed up on stage to provide first aid.
The 911 call came in as a seizure, but no one really knows yet. An ambulance came and paramedics tended to Heather.
Heather tells TMZ ... she felt somewhat dizzy when she was on stage and then just collapsed. She says it's never happened to her before.
She passed on the ambulance ... she says her sister-in-law -- who was in the audience -- took her to the hospital.
Heather says she got a cat scan as well as other tests, but so far it's unclear what caused her to faint. She says her head really hurts and she's very tired.
Ever the trooper, she says she feels horrible about abruptly ending the show.
Heather tells us she never had COVID and, as she said in her set, she's fully vaxxed and boosted.