The 23-year-old former TOWIE star said her obsession with keeping fit was eventually pointed out her to by a stylist, after she'd got down to a tiny size six.
Speaking to Fabulous magazine, Lucy said: "She told me I was too thin and said I needed to put on a bit of weight.
"Until then, I think people were a bit scared of saying anything to me. But I can see now how girls can get eating disorders, because being thin made me feel good.
"I don't think at the time I realised how thin I was."
Following her split from Mario, 26, Lucy hired a personal trainer and soon exercise was taking over her life.
She continued: "I was addicted to exercise. It wasn't about being thin, but it made me happier to do it so I'd do it a lot more. After than shoot I went shopping and properly looked at myself in the changing room mirror.
"I thought, 'OK, I'm too thin. I need to put on weight'. So I made a conscious effort to eat a bit more and not exercise as much."
While Lucy has now cut down on exercising and tries to eat a lot more, she hasn't totally given it up.
She added: "I work out to stay fit and lean, not to lose weight."