"Sad... As if we didn't have enough people glamorizing eat disorders already," the 21-year-old tweeted Friday. "Bottom line, it's not 'cool' or 'artsy' at all."
"Would you let someone bring a needle and shoot up on you? Addiction is addiction," she added. "It's not 'shade' and it's not 'hate.' But someone has to come forward and say it and I'll take the heat for it."
Another Twitter user pointed out that Lady Gaga has previously struggled with an eating disorder, and discussed it before the performance, but Lovato said that didn't matter.
"So it's cool if I do a speech about self-harm and then proceed to watch someone cut themselves on my stage? No. That doesn't make it OKAY," she wrote. "Putting the word ART in it isn't a free card to do whatever you want without consequences."
"More than any of you know, and I sympathize for everyone's struggle," she said. "But people emulate what they see celebrities do or let happen. And that's why I had to say something.. to let the people who don't understand the art in it, that bulimia isn't cool, and it won't get you on stage with your favorite artist. I can't NOT say anything because I stand up for what I believe in and speak out about mental illnesses."
Millie Brown, the performance artist who puked on Gaga, told TMZ on Saturday that their show was about non-conformity.
"There's a clear difference between using my body to create something beautiful, to express myself and feel powerful, rather than using it to punish myself and conform to society's standards."
Notable deaths of 2014 [PHOTOS]