The singer, 29, recalled she "couldn't speak" when she came face-to-face with the singer during a chat with bandmates Jesy Nelson, Jade Thirlwall, and Perrie Edwards on Saturday's (10Oct20) episode of their talent contest "Little Mix: The Search".
She made the revelation as the girls discussed how they still get overwhelmed when they meet celebrities, with Perrie saying she "had never been such a mess" as when she met actress Kate Hudson.
Leigh-Anne chimed, "For me it was Rihanna," as Jesy recalled, "Oh when she said 'I like your hair' and you couldn't even speak!"
"I couldn't speak," the "Holiday" singer agreed before doing an impression of her mumbling when she met the "Umbrella" star.
Perrie then said, "I don't think we'll ever be cool will we and not freak out when we meet people we love!"
The bandmates then discussed their experiences at various BRIT Awards bashes as Jade admitted they "usually stick to themselves" and just sit in the corner "eating the free bread."
"We're like four fan girls sat there at the BRITS, like we won tickets to be there," laughed Perrie.
While Little Mix didn't receive any nomination at the latest Brit Awards, they took home multiple trophies in the previous years. They won British Video of the Year for four consecutive years and British Single of the Year for three consecutive years since 2016. The girls were additionally named the best British Group twice in 2017 and 2019.