The 26-year-old recently showed off her new body on the cover of the US magazine after losing 50 pounds.
Speaking about the cover in her column in UK magazine Closer, she said: 'When I saw the photos for my Shape cover shoot, I burst into tears.
'I honestly never thought I'd be the girl wearing a bikini on the front of a magazine - especially considering that throughout my whole life so many people called me fat and ugly.
'I was so nervous back in September when we did the shoot - walking into a room full of bikinis is pretty overwhelming when you've never worn one!
'But it turned out to be an amazing experience and I even got teary when I saw the first shot of the day on the photographer's computer screen. I'm so pleased with the pictures - I just honestly cannot believe it's ME!'
The star, who recently sat down for an interview with Piers Morgan for an episode of his Life Stories show, also dismissed claims the cover was airbrushed.
She said: 'Predictably, the first thing people say when they see my Shape photos are: 'It has to be Photoshopped!' Which isn't true.
'Shape is an American fitness magazine and they have a policy of minimal - if any - airbrushing. It's about looking healthy, not flawless. I actually thought they might airbrush me a better six-pack, but I'm not complaining!'