In an interview with British GQ published Thursday, Butler revealed he was hospitalized the day after filming for the musical biopic concluded.
"The next day I woke up at four in the morning with excruciating pain, and I was rushed to hospital," the actor said.
Butler was reportedly diagnosed with a virus simulating appendicitis symptoms and was bedridden for a week.
He chalked it up to a physical reaction to not being in character as Presley. "My body just started shutting down the day after I finished 'Elvis,'" Butler said.
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The Disney alum claimed many people noticed that he couldn't shake off the King of Rock 'n' Roll's persona, and fans have commented on how his voice sounds since filming.
"My family said I didn’t sound like me anymore," he said as one example.
As soon as he was medically cleared to fly, Butler set out to London where he was quarantined before filming "Masters of the Air," a war-drama miniseries based on the Air Force serving in World War II.
Despite the elapsed time between filming "Elvis" and "Masters of the Air," he found himself still emulating the famed Graceland singer.
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"I was like, 'This is what Elvis felt when he was put into the Army,'" Butler recalled. He said he began imagining the role from the perspective of someone with glamour used to "hearing screaming fans, and then suddenly you’re just dressed like everybody else in those fatigues."
"Masters of the Air" director Cary Fukunaga added: "I was aware when he showed up, he was still very much Elvis."
Speaking of his admiration for Presley, Butler told British GQ, "I've never loved somebody I've never met more than Elvis."
"His mother passed away when he was 23, and my mom passed away when I was 23. So when I learned that, it was one of those things where I got chills and I just thought, 'OK, I can connect to that,'" he said.
"Elvis" debuted at Cannes film festival on Wednesday.
Butler walked the red carpet with Presley's ex-wife Priscilla Presley, who has given the stamp of approval of the film, and there was an emotional, 12-minute standing ovation after the movie's premiere.
"Elvis" director Baz Luhrmann told reporters at the Cannes film festival the Presley family gave the movie the "greatest review" of his life.
"Elvis" debuts in theaters June 24.
This article originally appeared on USA TODAY: Austin Butler hospitalized after 'Elvis' filming ended, he says