She later commented on the photo, however, and in what appeared to be a response to her hair stylist Scotty Cunha wrote "ugh I was so fat."
Fans were quick to react to the comment.
"You weren't, and even if you were, who cares?" wrote one. "How many young girls follow you and have now seen this? Sigh."
“Extremely disappointing to see someone that I consider a role model refer to themselves as ‘fat’ on a public platform where other girls can also see it,” wrote another.
“Really disappointed that you would use this type of language knowing young girls look up to you. Esp because you were nowhere near overweight you may reinforce harmful thoughts in these impressionable minds," said another.
"Lost all respect for you after calling yourself fat in the other picture.... honestly I'm double your size and work my ass off daily so what am I to you????" said a disgruntled fan.
While some comments later defended the star's right to say she "felt fat" if she wanted to, others pointed out that this comment was worrying due to Lucy's previous weight battles.
In 2014 the star revealed she had suffered from eating disorders, telling Cosmopolitan:
"I’ve never really talked about this, but I would go days without eating. Or maybe I’d have some fruit and then go to the gym for three hours. I knew I had a problem…It was a gradual process but I changed myself."
Lucy removed the comment function from this particular image, before taking to Twitter to clear the air.
"Sometimes I forget that people are watching everything I do and I say things I don't mean," she wrote, before apologising to fans.