Katy Perry** has been called "The First Lady of Hot" by Maxim, but she doesn't always feel worthy of the title.

Us Weekly caught up with the "Part of Me" singer at the Nov. 28 UNICEF Snowflake Ball in New York City, where she and other celebs including Selena Gomez, Uma Thurman, Tony Bennett and Kelly Ripa walked the blue carpet and helped raise money for UNICEF'S lifesaving children's programs.

The pop star, 28, was stunning in a tight, floor-length, semi-sheer navy Naeem Khan dress with a matching cape, Guiseppi Zanotti heels, a Judith Leiber clutch, and Lorraine Schwartz jewels -- but she admitted she didn't feel as good as others thought she looked.

"I feel like my [fat] is sticking out so bad right now," she confided in Us at the bash. "I almost always wear [Spanx], but not tonight, and I feel it pushing out, making me look fat."

"I like to say I run on the elliptical for 30 minutes watching CNN, but I don't do it often enough," she added. "So I'm just like, 'Oh, I guess I'm not fitting into this Lanvin dress for these couple of months.'"

Perry may not always love her curves, but plenty of other people do -- including on-again boyfriend John Mayer**, who recently accompanied the singer on a romantic tour of the Old Mission in her hometown of Santa Barbara, Calif. The two have also been spotted packing on the PDA at several Hollywood hot spots.

"They were in their own world," a guest at Perry's October birthday bash told Us of the couple, who reunited in September after briefly separating in August. "She didn't even talk to her friends much. It was just her and John. She's obsessed with him."

This article originally appeared on Usmagazine.com: Katy Perry: I Feel Fat Without Spanx!