The 27-year-old fashion model, who shot to fame in 2005 on TV's popular Make Me a Supermodel, allegedly had a six-month fling with the 52-year-old X Factor boss.
Discussing the relationship, Lennard vowed that "the truth will come out in the end".
"I don't have anything bad to say," she told Coleen Nolan. "That person was such a huge part of my life. I don't want my life to be defined by it.
"The way the relationship has gone down in history is the complete opposite to how it went down."
She added passionately: "I had an affair with Simon Cowell and I do really want to talk about it.
"Rich people get to keep secrets. They have the money to stop you and I just don't think it's fair. It pains me, it angers me, it really frustrates me."
Celebrity Big Brother continues tonight (August 17) from 9pm on Channel 5, with live nominations following at 10pm, and Bit on the Side airing from 10.30pm.
Photo gallery - Celebrity Big Brother day 2:
Copyright: Channel 5