The 59-year-old matriarch has never felt so red-faced as she did after an American Airlines flight attendant announced to the other customers over the tannoy that she and the 65-year-old TV personality, formerly known as Bruce Jenner prior to her transition surgery, had joined the Mile High Club.
Speaking in Cosmopolitan magazine's 50th anniversary issue, she said: ''The most embarrassing moment was when I was with [Caitlyn] on a commercial airliner in first class. We joined the Mile High Club and felt we got away with it.
She added: ''We had sex in the bathroom and we came out and nobody said anything.
''At the end of the flight, the flight attendant got on the microphone, 'Congratulations, Mr. and Mrs. Jenner! You've just joined the mile-high club. We're so proud of you, and we decided to give you a bottle of champagne! Yay!' I could not squish down in my seat low enough. I was mortified.''
Meanwhile, the 'Keeping Up with The Kardashians' star recently told her ex-husband to go ''f**k himself'' after she accused her of bullying her during their 24 years of marriage in her first interview as a woman with Vanity Fair magazine.
Kris fumed: ''It just sums up that I'm this piece of s**t person who was mean. He can go f**k himself. I honestly wish I never met this man.''
The former couple married in 1991 and have two daughters together, Kendall,19, and 18-year-old Kylie, but separated in 2013 and divorced two years later.
'Smalls Change (Meditations Upon Ageing)' arrives in April.
The two awards have made for a great 72nd birthday present for the country music icon.