The 18-year-old singer - who was treated for an eating disorder at an unnamed Chicago-based facility between last November and January, and has been diagnosed as bipolar - has revealed she was cruelly teased by other children when she was at school, and she insists the traumatic experience played a part in her dramatic weight loss.
Speaking on TV show `Good Morning America`, Demi said: "I literally didn`t know why they were being so mean. I was bullied for being fat."
She added that she had virtually "stopped eating" at the age of 12 - consuming only two meals a week and dropped 30lbs, to weigh 95lbs.
A life coach was brought in to help the star, who said: "I thought it was enough, so everyone else thought it was enough.
"In reality I was only eating a meal a day. When I was about 15, I was only eating two meals a week, but I wasn`t losing any more weight because my body adjusted to that. So I tried new things, laxatives, fasts - nothing was working."
Demi also turned to bulimia, adding: "At my worst, I was doing it five times a day. I threw up so hard and so much, it was just blood in the toilet."
The star quit her popular TV show `Sonny with a Chance` earlier this month because she was nervous stepping in front of the cameras again, but she is adamant her Disney bosses never asked her to lose weight.
Demi also revealed her eating disorder led her to self-harm as "a way of relieving pressure" when she was "stressed and had anxiety".
She is now taking her recovery "day by day" and concentrating on her musical career, saying: "It made sense to me to leave the show and focus on my music. In the studio you don`t see my body, and I feel all of my confidence is in my voice.
"I don`t know if I could handle being in front of the camera with my body right now."