The Love Actually star revealed the Duke of Cambridge had spiked her on the chest while she received her Damehood.
On the Ellen DeGeneres Show, the actress recreated the moment with host Ms DeGeneres.
She showed where William placed the plain causing Ms DeGeneres to exclaim, “oh God”.
Ms Thompson said: “Exactly! And it’s a little bit of a nipple moment and you go, ‘Gosh, I wasn’t expecting that.”
She added the second pin went lower.
She said: “And I’m pretty low slung these days, so both times it hurt!”
“I made a very loud and inappropriate noise in Buckingham Palace. It’s not a surprise, is it?”
Dame Thompson went on to say she received backlash for wearing white Stella McCartney to the ceremony.
People online commented saying the shoes were “too casual” for the occasion.
Ms Thompson noticed Ms DeGeneres was wearing the exact same trainers while on the show.
She said: “They were posh trainers.
“You get a big old badge, you get two badges, and I was wearing exactly those shoes and they gave me such hell.
After the ceremony, Ms Thompson said: “I love Prince William, I’ve known him since he was little, and we just sniggered at each other.
“I said, ‘I can’t kiss you, can I?’ And he said, ‘No don’t!’
“If you’re first up you have to be more formal on such an occasion, but he’s looking wonderful and doing so well.”