Sydney Sweeney’s body underwent major transformations both before and after filming the boxing biopic,Christy.

The actress famously gained over 30 pounds to believably embody boxer Christy Martin by changing her diet and prioritizing strength training over a matter of grueling months. When shooting wrapped on the film, directed by David Michôd, Sweeney tells PEOPLE she only had seven weeks before stepping onto the sets of two very different projects:The Housemaidand the third season ofEuphoria.

“I had to be really strict with myself,” Sweeney, 28, recalls of how she lost the weight she had gained forChristyunder a short timeline.

“When I was training, it was really hard to put on all the weight. I was eating so much food. Like, everything you could possibly imagine,” she continues. “And then I was downing protein shake after protein shake and taking a lot of creatine, which just makes you bloat.”

To return to her pre-Christyfigure, Sweeney says she “stopped all of that.”

“When you stop working out and taking all your protein shakes, you lose muscle super fast,” she says. “It’s the first thing to go before fat. So, that, I dropped within two weeks.”

The rest of her weight loss journey involved a “super clean” diet and “a lot of cardio,” theWhite Lotusactress explains.

While Sweeney says “this might be the one and only” time she challenges her body this strenuously for a role, she’s grateful that her “metabolism works really well.”

“I don’t want to say this is super easy,” she stresses regarding the weight loss. “But I’ve been a very active person my entire life. Like, I was a super active kid, and I’ve always kind of just stayed active and always had a good metabolism. I can gain weight, but I can lose it too.”

Christyis now in theaters.The Housemaidcostarring Amanda Seyfried, opens in cinemas on Dec. 19.