Millie Brown was 17 years old, alone on a Berlin stage, about to try something she had never done before. Beside her were seven bottles of milk, each one dyed with a different blend of food colouring. She composed herself and drank the first one, red.

"I wanted to use my body to create art," she remembers. "I wanted it to truly come from within, to create something beautiful that was raw and uncontrollable." So what exactly did she do? "I vomited the entire rainbow on the stage." Altogether, it took around two hours, colour after colour. "I wasn't even sure if it was physically possible," she says. "I hadn't eaten in days because I didn't know how long it took for my stomach to clean out food. And I don't think having chunks of food within the paint is necessarily beautiful."

A Londoner, Brown had been invited to Berlin as part of Peckham's !WOWOW! collective. She'd recently become interested in performance art, and this new piece, vomiting on to canvas, made a deep impression on the audience. "There was actually an old lady who was so moved she left crying," she says. Nine years on, her work is shown in galleries and she has experienced a wide range of reactions to it: anger, sympathy, laughter, elation, disgust. There are also accusations that she glamorises bulimia, following her new collaboration with Lady Gaga, a friend since they made a short video together in 2009.

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During Gaga's performance of Swine at the SXSW conference in Texas, Brown drank a bottle of luminous green milk, which she then vomited on to Gaga's breasts, before drinking a bottle of black milk, climbing onto a mechanical bull with the singer, and doing it again, the two of them writhing in the mess. "Puke isn't art, you ugly whore," is one of the messages Brown has since received on Twitter. "You are a piece of shit like Gaga," is another. Many others, probably the majority, expressed support. Some even asked to be vomited on.

The reaction from the media has been more jaded. We've already had Artist's Shit after all, and Piss Christ, and Marc Quinn's Self made with his own blood, and Tracey Emin's My Bed and its medley of stains. When you hear about Brown, it's natural just to shrug and say: well, a cheque was waiting for whoever ticked the vomit box.

But if you watch a clip on YouTube, you'll probably find it quite unsettling. While Gaga shouts, "Fuck you, pop music! This is artpop! Free yourself!" before her second drenching, Brown is mute and neat, almost timid in her movements. She goes about the retching process with a kind of demure determination that is hard to watch, and even after all these years she doesn't seem very good at it. Those two middle fingers rummage around her throat for ages until they find the switch, and often release only a dribble. "The struggle makes the performance," she says. "I think it's very human." It certainly isn't glamour; it's not even Gaga's anti-glamour. It's sad, undignified and real.

Some people strongly disagree. "Bulimia isn't cool," tweeted the singer and actor Demi Lovato, a former sufferer. "Young people who are struggling to figure out their identities are seriously influenced by the things they see their idols do." Gaga, once bulimic too, pleads artistic licence. "I'm not saying vomit is going to change the world," she told the Texas audience. "What I'm saying is it's … just what we wanted to create." Nevertheless, an online petition is now calling for Gaga to end her association with Brown. At the time of writing, it had just passed 10,500 signatures.

Brown on stage with Lady Gaga at SXSW. Photograph: Fuse/Xposurephotos.com

How does Brown feel about this? "I'm using my body to create something beautiful," she says. "I think it's misunderstood by a lot of people. But it really doesn't have anything to do with eating disorders. If I was male, [no one would make] such a massive association." Indeed Brown, who has never had an eating disorder, is careful about her health. Following one reprise of the rainbow at the Chapman brothers' house, she now restricts herself to a less gruelling palette of just a few colours, and takes a month off in between performances, concentrating on other types of work. "I have experienced migraines," she admits. "But I find that if I just do two or three colours, my whole body feels completely fine afterwards."

Yet she does suffer in other ways. Now based in LA, she became a vegan about five years ago, partly because she'd grown disgusted by cow's milk (since replaced with soya in her shows). Later, the disgust turned into fear, giving her what she calls "a really strange phobia". Having once drunk litres of milk professionally, she now can't touch a carton, or be happy in the same room as a cappuccino. "Even the thought of it makes me feel sick," she says. When I suggest this may be a Pavlovian consequence of her work, the idea seems new to her. "Maybe," she says. "Maybe I'll be completely repulsed by soy milk one day."

My final question is delicate. Brown has told me that quite a bit of fluid stays in her after each performance. So does she end up creating further works of art in the toilet? She laughs. "Actually yes," she says. "That's more of a private performance for myself."