The 38-year-old singer made the candid confession in a new chat in which she also described her time working with the music and talent show mogul as having gone on "longer than" it should have.
Lewis had become the nation's sweetheart when she won Cowell's show, The X Factor, in 2006 and then went on to become a global superstar.
Signed to Cowell's label, Syco, her debut single Bleeding Love went to number one in 35 countries when released in 2007, leading to her first record Spirit becoming the UK’s fastest-selling debut album of all time and hitting number one in the US.
The star, who was the first British artist to achieve that with a debut, has gone on to sell over 35 million records with Spirit and her subsequent albums.
But she hasn't released a studio album since 2015’s I Am.
In February of 2014 Lewis posted a handwritten letter on social media explaining why she left Syco after the label claimed publicly that she'd been dropped.
She wrote, “At some points I felt extremely depressed… it got to a place where the downs were outweighing the ups… I cannot make music that does not speak to my soul, and as scary as it seemed, I could no longer compromise myself”.
“When there’s a lot of cooks in the kitchen, things can get messy,” she told the inews of her time with Cowell and the Syco team, on Monday.
“There was a lot of miscommunication at the time. It was very upsetting for me. And it really hurt. And I think my fans wanted to know, ‘What’s going on?’ I just wanted to be very, very open, very honest. You know when you’re in a relationship longer than you should be? It was that.”â¯â¯â¯
She added of her relationship with Britain's Got Talent boss, Cowell, “we’ve had ups and downs”.
The pair are now on good terms. Lewis, who shares one-year-old daughter, Carmel with her choreographer husband, Dennis Jauch, even recently sold her LA mansion to Cowell at "a discount" for a reported $3.9m.â¯
In January, the singer starts filming a semi-autobiographical musical drama set in a school in Hackney, which she co-wrote with actor Yolanda Mercy.
Lewis will also hit the stage to perform for her gig Leona Lewis: Christmas with Love on Tuesday 28 November at the Royal Concert Hall in Nottingham.