Ally Hilfiger bursts into her publicist’s office and does a full twirl, showing off the outfit she filched from her mother’s closet that morning.

“Vintage Gucci,” she chirps, pointing to her sensibly heeled shoes. Tailored pants, a black turtleneck and a Catherine Walker jacket give her a professional air, but her boundless banter is that of a confidante.

This story first appeared in the May 11, 2016 issue of WWD. Subscribe Today.

“Mom’s a size 2 and I’m a 2,” she says. “I used to be a zero. Now, I’m 10 pounds heavier. It’s like Marilyn Monroe — I feel sexier, womanly and strong.”

Strong is a revelation for Hilfiger. The daughter of designer Tommy and his first wife, Susie, she suffered from joint pain, night sweats, memory loss, nausea and brain fog caused by Lyme disease, which went undiagnosed for more than a decade.

During the course of a recent interview, Hilfiger is elated, angry and sad, but never bitter. Her account of the ordeal, “Bite Me: How Lyme Disease Stole My Childhood, Made Me Crazy and Almost Killed Me,” was published May 10 by Center Street, a Hachette Book Group imprint.

When the first copy of “Bite Me” arrives via messenger, Hilfiger begins to cry. “Oh my gosh! It’s the first time I’m seeing it,” she says. “Oh God! I put my heart and soul into this thing. It’s a heavy book. It’s big! Wow! What does it smell like? It smells good.”

The memory of her mother plucking a black tick from her belly when she was six or seven years old still looms large in her imagination. Hilfiger says she was probably bitten in Nantucket, Mass., or Bridgehampton, N.Y., where the family summered, or Greenwich, Conn., where their main residence was at the time.

It would be easy to dismiss Hilfiger as just another poor little rich girl trying to parlay her misfortune into a few incremental rungs on the ladder of fame, except she beats you to the punch.

“I do fall into the stereotype of a privileged white person on the East Coast coming down with Lyme disease,” she says. “It doesn’t discriminate against race, skin color or gender. It doesn’t care what kind of shoes you wear or what kind of car you drive. It’s a cruel disease. I was disbelieved. The doctors said, ‘You have growing pains, you’re fine.'”

Finally, Hilfiger says she simply “zipped my lips. I didn’t want to come across as a complainer.”

She woke up most mornings exhausted and stumbled through school in a constant state of confusion. Medical experts tagged her with everything from rheumatoid arthritis to fibromyalgia.

During high school, Hilfiger sold MTV on an idea for “Rich Girls,” an early reality TV show, which she produced and starred in with her then-best friend, Jaime Gleicher. The show, which aired in fall 2003 and winter 2004, opened with the women saying, “People think we have it all together.” As cameras followed them to London, Los Angeles and the British West Indies, they seemed to have it all together, but Hilfiger was actually falling apart.

As her physical health deteriorated, she drank and smoked pot to ease the aching in her joints.

After a public breakdown on “Rich Girls,” Hilfiger says her father took her to Silver Hill Mental Health Hospital, in New Canaan, Conn., where she awoke from a drug-induced torpor confused to find herself in an addiction rehab facility. She says she took comfort in the fact that Liza Minnelli, Truman Capote and Gregg Allman had done stints there.

“It was a full-on breakdown,” she says. “The Lyme disease and stress of the show had gotten to my brain. I was so confused about how [my father] could lock me up somewhere, but my behavior wasn’t normal. I was yelling, ‘There are bugs inside of me.’

“I thank him, I don’t blame him,” Hilfiger says of her father, who wrote the forward to “Bite Me.” After all, it was at Silver Hill that she met Dr. Ellyn Shander, who finally identified her illness as Lyme.

Hilfiger’s lost childhood is a recurring theme. “Do you know how many therapists have told me how much of my childhood I missed? I really didn’t have an adolescence,” she says in an episode of “Rich Girls.”

There’s still lingering regret. “I feel very sad for that little girl and sad for her parents,” she says.

Hilfiger eased back into the real world by working at her mother’s Greenwich store, Best & Co. Then, with her father’s backing, in 2011 she founded Nahm with designer Nary Manivong.

A year later, Hilfiger decided to move to Los Angeles for her health. Closing Nahm “was devastating,” she says.

Hilfiger would one day like to “bring balance and good health to the fashion world.” She is feeling healthy these days, but works hard to stay so by eating and living clean.

Now 31, Hilfiger is focused on being a mother to Harley, who was born last year, and raising awareness of Lyme disease. “I think I got sick so I could share my story,” she says.