When Johanna Poremba went to the doctor with abdominal pain last summer, colon cancer was the last thing on her mind.

“I had been vegetarian for years now. I work out and I don’t really drink or smoke at all,” Poremba, 22, of Hampton Bays, said. “We thought it was a lactose allergy.”

But even after cutting out dairy, Poremba said the pain worsened. It was then that her doctor performed a colonoscopy and discovered a tumor.

The diagnosis shocked Poremba, who said she doesn’t have a family history of colon cancer. Because of her age and healthy lifestyle, cancer — let alone cancer prevention — wasn’t a concern.

Poremba has since used her experiences to spread awareness, especially to tell young people that they, too, could be at risk. Now she’s taking her story to “The Dr. Oz Show.”

Though colon cancer has declined in older age groups, a recent American Cancer Society study found it is more prevalent in young people than previously thought and continues to rise.

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The 20 to 39 age group has seen some of the largest increases in colon cancer of any age group, between 1 and 2.4 percent annually since the mid-1980s. Some scientists have said the spike could be generally tied to diet trends or a sedentary lifestyle, but no definitive cause has been found.

Dr. Louis Avvento, who treated Poremba’s cancer at New York Cancer & Blood Specialists in Southampton, said there’s an increased concern about cancer risk in younger people.

“In the past 2-3 years, I’ve been seeing a lot of younger people with colon cancer — she’s the youngest so far,” Avvento said.

More doctors are encouraging their young patients to get colonoscopies when they have any possible symptoms, Avvento said. Warning signs include rectal bleeding and a severe change in bowel habits.

“Everyone gets a colonoscopy now, we don’t take for granted that it’s a hemorrhoid or anything,” he said.

In early March, Poremba posted a photo of herself on Facebook and Instagram holding a poster with information about colon cancer to mark the start of Colon Cancer Awareness Month. The post received nearly 800 likes on the two social media platforms and caught the attention of a “Dr. Oz” staff member, she said.

Poremba was invited to the set and filmed a segment on colon cancer in young people with another young female colon cancer patient at the show’s Manhattan studios on March 10.

The experience was exciting, she said, and she hopes her segment will inspire others to be more proactive about their health.

Poremba’s doctors removed the tumor in November and she is now receiving chemotherapy at New York Cancer & Blood Specialists in between attending law classes at Touro College. She’s also made a number of changes to her diet, including going vegan.

A “Dr. Oz” spokeswoman said the episode featuring Poremba will air Tuesday.

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“I just feel like I have this story to tell now,” she said. “I’m definitely excited to see it. I’ll probably have friends and family over for some vegan snacks.”

CORRECTION: A previous version of this story misstated the extent of Dr. Louis Avvento’s involvement in Poremba’s case.