The socialite admits she and the sportsman had spent a lot of time discussing the possibility of getting wed but she didn't think he would propose this year as she didn't think he'd want to upstage her mother Kris Jenner's 20-year anniversary with husband Bruce Jenner.
She told talk show host Jay Leno on 'The Tonight Show': "I knew in month two. We talked about it [marriage] so much, I was kind of like, 'Is this happening here?' But it was my mum and Bruce's 20-year anniversary, so I was like, 'He's not going to outshine them.' "
The brunette beauty insists the proposal was a total surprise to her - and she didn't react in the way she had expected.
She recalled: "We were in Los Angeles and it was the most random day. I come home and no one was home, his car wasn't there, nothing was there. And I opened the door and he was there and all the furniture in my bedroom was moved and I have this big white rug and it said, 'Will you marry me' in red rose petals.
"I walked in and there were candles everywhere and he was there on one knee. You envision this moment your whole life and you think, 'Oh my god I'm going to cry or I'm going to scream,' but I opened the door and I was like huh... Straight away I was like [intake of breath], [high pitched] yes!"
Jay also played a trick on Kim, swapping her $2 million engagement ring for a fake, before setting it on fire to test if it was real.
The fake ring he had swapped burst into flames, and Jay joked to his reality TV star guest: "That's not good... get the pre-nup."