The singer also wore geeky glasses and a hat as part of a fake TV advert.
However, the bulky outfit proved hard to dance in and Gomez fell over.
The 'producer' of the advert was then rude to Gomez's fan, Mary, before the star revealed her identity and told the fan it was all a prank.
“Mary was such a good sport,” said Gomez at the end. “She was so nice, so it was really hard for me to keep up the act when Kara was being so mean to her. It was very fun to prank her and I think she did a very good job.”