The 30-year-old pop star was given oxygen after collapsing in the Glastonbury VIP area at Worthy Farm, reported The Daily Mirror.
"She was out of it, she didn't know where she was. Her friends were saying, 'You all right?' and trying to give her water," an eyewitness said.
The "Smile" hitmaker was treated by paramedics when her friends picked her up and placed her on the back seat of a nearby car.
Allen, mother of two kids, was given oxygen before husband Sam Cooper eventually drove her away.