Celebrity Big Brother star Katie made the confession during last night's episode, saying she is so haunted by the New Year's Eve bash that she cannot bear to don an all-in-one again.
Speaking to her shocked housemates, Katie revealed that she spotted Kieran and former best friend Jane Pountney on a security camera as they headed off to her stables together.
She said: 'Whenever I do parties they are always fancy dress, because I think people make an effort.
"This was the first year I didn't because, last year, I did a onesie one and Keiran and Jane f***ed in my stables so I will never wear a onesie again.
"It was on the cameras, it was 1am on my cameras. You can them walking up to the stables and I never knew, everyone was in my house partying."
Since making her surprise arrival in the CBB house on Friday, Katie hasn't held back when talking about her marital problems with third husband Kieran.
On the weekend, Katie told Katie Hopkins and Calum Best how she caught Kieran and Jane kissing on a sun lounger while on holiday in Cape Verde.
She said: "I got her f*****g head, f*****g pulled her off the sun bed, f*****g whacking her." Katie then revealed that Jane and Kieran had been "s******g for 10 months".
Katie first made Kieran's affair public last May, however, after vowing to divorce him, the couple have worked through their issues. Kieran, meanwhile, has received counselling for a 'sex addiction'.
Earlier today, we revealed how Katie now "trusts Kieran 100 per cent" and isn't worried about him straying while she is away.