Monnin resigned her Miss Pennsylvania title earlier this week after claiming that Miss Florida Karina Brez told her prior to the competition that she'd seen a list of the top five finalists, thus insinuating that the competition was fixed.
Brez has since denied ever actually seeing such a list, but Monnin kept up her attacks on Miss USA by calling the organisation "fraudulent" and "trashy" in a Facebook posting.
Monnin appeared on NBC's Today to respond to Miss USA owner Donald Trump's promise to sue her if she didn't issue an immediate retraction.
"I feel disappointed that (Trump) has made some of the statements that he said about me, and I feel prepared to continue to pursue the truth," Monnin said.
"I know what I heard, and I know what I in turn witnessed come true based on what I heard the contestant said she saw on the list, so I'm prepared to continue to march forward."
"I know what I heard. There's no doubt in my mind that the contestant was serious when she laid out what she said she saw, and I believe her to be true," she said.
Monnin continued: "I have many years of psychological training. I know when someone's telling a joke, I know when someone's scared and when someone's serious, and in my opinion her body language was very serious.
"[Brez] looked a little bit scared because she had just seen something that would potentially, drastically change the reputation of the Miss Universe Organisation. This is a big deal!"
Miss USA has denied any claims that the competition is fixed and suggested that the politically conservative Monnin actually quit because transgender contestants are now allowed in the pageant.
Olivia Culpo, who represents Rhode Island, was crowned Miss USA 2012 on Saturday in Las Vegas.
Monnin finished outside the Top 16.
Watch Sheena Monnin's Today interview below: