Speaking to Us Weekly at the UNICEF Snowflake Ball on Tuesday (November 28), the 'Part of Me' singer confessed that she was feeling self-conscious about her outfit.
"I feel like my [fat] is sticking out so bad right now," she said. "I almost always wear [Spanx], but not tonight, and I feel it pushing out, making me look fat."
Perry also confessed that she does not exercise as much as she should, saying: "I like to say I run on the elliptical for 30 minutes watching CNN, but I don't do it often enough. So I'm just like, 'Oh, I guess I'm not fitting into this Lanvin dress for these couple of months'."
Perry, who is currently dating John Mayer, is reportedly working on her third studio album.
She was recently criticised by Crystal Castles frontwoman Alice Glass, who alleged that Perry "sells sex to children".
Watch the video for Katy Perry's 'Wide Awake' below: