The 56-year-old was pulled backwards off stage by a cape, which was supposed to fall from her shoulders.
In an interview on The Jonathan Ross Show, taped on Thursday, the star said: 'I had a little bit of whiplash, I smacked the back of my head. And I had a man standing over me with a flashlight until about 3am to make sure I was compos mentis.'
She added: 'I know how to fall, I have fallen off my horse many times.'
The singer blamed her fall on a wardrobe malfunction, saying her cape had been tied too tightly, meaning it could not be unfastened in time.
'Everyone was worried my cape would slide off, so they tied it really tight around my neck,' explained the singer.
'I got to the top of the stairs and I pulled the silky string, and it wouldn't come undone. I had two choices: I could either be strangled or fall, and I chose to fall.'
The star recovered and finished her performance, which was the finale of the Brit Awards - where Ed Sheeran and Sam Smith were the big winners.
She refused to watch a replay of the incident on Jonathan Ross's show, which will be broadcast on ITV on Saturday 14 March.
'It was a nightmare,' she said, 'I actually created a horror show for everyone.'
The star added that she would not be recreating the performance on her forthcoming tour.
'No more capes. Cape fear is over.'
From BBC News