The Hot Right Now singer is noted for her head-to-toe approach to expensive labels but admitted she likes to nip into the cheap and cheerful firm's shops to stock up on clothes when the need takes her.
In an interview with the new edition of Top Of The Pops magazine, she said: "Sometimes I put on a hoody and shades and go into Primark [Penneys in the UK].
"Last time, I bought underwear and socks cos I was on tour and I'd run out of everything."
Incidentally, she is currently featuring in a campaign for retailer Marks & Spencer alongside Annie Lennox and Alek Wek.
The performer also talks about recording with her boyfriend, the producer and DJ Calvin Harris with whom she worked on new single I Will Never Let You Down.
"I felt really shy working with him. It was an experience, but I was nervous," she told the magazine, which is out tomorrow.
"It's like any relationship. I just want to stay there with him and I can be totally relaxed. He's my best friend."