The singer admits that she and her cohorts on the panel – Randy Jackson and Steven Tyler – recently had their most combative disagreement since coming together as a threesome on last season's show.
"Me and Randy and Steve got into our first fight. I got really, really upset," Lopez, 42, told Ryan Seacrest's KIIS-FM radio show Friday.
The blowout – which Seacrest confirmed visibly riled her – bubbled up at a recent audition after a girl with what Lopez felt was an exceptional voice performed for the panel.
"She was amazing," Lopez recalls.
But Tyler didn't agree – and asked for another song.
"I was surprised because I was ready to vote," Lopez said, adding with a laugh, "Sometimes the days get kinda long."
When the girl's second song got going, though, Tyler, and then Jackson, quickly nixed the girl from going further in the competition.
"I thought I was being Punk'd," Lopez recalls. "[I thought], 'How can we let this girl go?' I was upset. When I believe in something ... I knew this girl deserves the chance."
All in all, though, Lopez says her second season is "going really great," but does admit: "The kids, we're being really tough on them."
Lopez, whose new single Papi (off her latest album Love) is now out, briefly referenced her soon-to-be ex-husband Marc Anthony.
"I call everybody 'Papi,' " Lopez said of the inspiration for the song. "I used to call Marc 'Papi.' We wrote the song about loving somebody and celebrating that."