"I'd been to every incredible party in the world," says Hilton, 30. "But I was going out with my girlfriends and dancing, having a lot of coffee and Red Bull. Being on the go, I would just grab fast food or skip meals because I was busy."
But that lifestyle drained her body and by last year, the 5'8" Hilton weighed just 105 pounds and fit into a size zero.
Hilton's mom, Kathy, took notice.
"She would be like, 'You're way too skinny. It doesn't look good,'" Hilton recalls. "I said, 'Mom, everyone is skinny in Hollywood. It's just the way it is.' And she's like, 'No. It's not normal.'"
Enter boyfriend Cy Waits, who also helped advise Hilton on getting healthy when the pair started dating last summer.
"He explained everything to me," Hilton says of Waits, a former nightclub owner. "Now I only eat organic foods when we're at home. He makes me watch documentaries on food, like Fast Food Nation, and he makes me aware of what things are good for your body because he's very into that."
By December, however, paparazzi thought Hilton was having too much of a good thing when she hit 125 lbs, and the pregnancy rumors followed.
"Even if I'm a few pounds overweight, people will be like, 'Oh, she's fat,'" says Hilton. "And it's not fair that it's like that, but it is."
Today, Hilton has settled in at 115 lbs., she goes jogging with Waits, and works with a nutritionist.
"I'm really happy with the weight I'm at," adds Hilton, whose show The World According to Paris airs Wednesdays on Oxygen. "I feel like it's healthy and normal … I'm just making smart choices."
For more on Paris's weight, bikini body and healthy habits, pick up this week's Most Amazing Bodies issue of PEOPLE magazine