The British singer - who was the face of the French brand's handbag range - admits she was not confident during the photo shoot because she doesn't have normal model looks.
She said: "When Karl Lagerfeld called about the campaign, I didn't tell anyone because I was convinced they made some huge mistake and that it wouldn't happen.
The 25-year-old star - who is currently expecting her first child with boyfriend Sam Cooper - also admits her shopping habits have changed since she became famous as she gets no privacy in high street stores.
She told Britain's Elle magazine: "My shopping habits definitely changed as I became more well known.
"I feel awkward now because people stop and stare and take pictures. That's probably why I'm more comfortable walking into YVES SAINT LAURENT and buying a handbag. That doesn't mean that I enjoy it more. Fame pushes you into designer stores because it gives you the luxury of privacy."