The 22-year-old worked with One Republic's Ryan Tedder, who wrote Leona Lewis' track Bleeding Love, on her upcoming album 21, which is due out on January 24.
She said to UK newspaper Daily Star: 'I loved working with Ryan. And he came cheap. I didn't pay him. He said he was a big fan and that was it. I'm very lucky.'
Asked how she met him, she said: 'I met him in a lift one night in LA. I was holding 100 balloons, so the lift was full, but he could hear me laughing from inside.
'He knew it was me because he recognised my Catherine Tate 'Nan' laugh. He got in the lift and started popping all the balloons and said we had to work together. We met again in London and I started telling him everything.'