Speaking to ABC's Robin Roberts, the former Sonny With A Chance star said that the bullying she experienced in her childhood contributed to her eating disorder.
"I literally didn't know why they were being so mean to me. And when I would ask them why, they would just say, 'Well, you're fat,'" Lovato said.
The 18-year-old added: "I was bullied because I was fat. And then a few months later I developed an eating disorder, and that's kind of what I've been dealing with ever since."
Lovato sought treatment for emotional and physical issues last year. She left a treatment centre in January.
A preview of Lovato's interview with Roberts will air Friday, April 22 on Good Morning America, with the full interview airing that night on 20/20 at 10pm ET on ABC.