The singer, presently on the European leg of her Circus world tour, was allegedly practising moves for a new song when she lost consciousness, and fell to the floor.
"She collapsed when the dancers went to put her back on her feet. She fell to the floor and landed on one of the dancer's legs," Contactmusic quoted a source as telling the National Enquirer. "The dancer's tennis shoe smacked Britney in the chest and on the face. It hit her chest with such a force that it caused a four-inch bruise," the source added.
The blame was laid on the 'Womanizer' htimaker's busy schedule, and a low blood-sugar level.