The 38-year-old fashion designer reteamed with her Spice Girls bandmates Mel B, Mel C, Emma Bunton and Geri Halliwell earlier this month for the premiere of their new West End show Viva Forever.
In an interview for upcoming documentary Spice Girls' Story: Viva Forever, Beckham talked about being nicknamed Posh.
She said: "I was called Posh because I liked the nice restaurants, and the nice clothes, and that was my character. I didn't smile, even in those days."
Beckham, who has laughed off criticism of her moody demeanour in the past, added:" There was this very strong image. And I am very much that person, even now.
"I get ready to go out sometimes and I catch myself in the mirror and I'm like, 'Oh my God, I've turned into that person'."
Recalling the band's heyday, she quipped: "The things we used to get up to! We had a lot of fun. Even me!"
Spice Girls' Story: Viva Forever airs on ITV1 in the UK at 8.30pm on Monday, December 24.
Viva Forever is currently running at the Piccadilly Theatre in London.
Photo gallery - Viva Forever premiere: