Her colon cancer grew undiagnosed for 15 years. Now, at 40, she's fighting back.

Dawn Eicher was 21 when she began complaining to doctors of classic symptoms of colon cancer — fatigue, abdominal discomfort, and bright red blood in her stool.

By the time her cancer was finally diagnosed 15 years later, soon after the birth of her second child, it had spread from her colon to her liver. Her prognosis was grim.

"Over the years, my symptoms would come and go, and disappear for six months," she recalled from her home in Kona, Hawaii, where she and her husband Michael are business consultants. "I kept going to different doctors. Every doctor missed it. I wish I had fought harder to get answers, but I didn't know. And I think everyone is taught from a young age that poop is something you don't talk about."

Eicher underwent debilitating surgeries, radiation and chemotherapy. Now 40, she does not take her nearly three-year remission for granted.

She has joined a growing number of shockingly young colon cancer survivors and patients in fighting the assumptions, deference, and embarrassment that almost killed her. She belongs to the Colon Cancer Alliance, Fight Colorectal Cancer, the Colon Club, Colontown and other groups.

"I have made it a huge goal in my life to educate the public about this disease," said Eicher, creator of a Facebook page for those who, like her, feel survivor's guilt for beating the odds.  "Colon cancer is on the rise in younger age groups. It's not an 'old man's disease' anymore, and yet it's one that most doctors don't discover in younger patients until it's at an advanced stage."

Colorectal cancer, which has been declining for decades among older adults, is rising sharply among Gen X and millennials, an American Cancer Society study revealed last month. For people in their 20s and 30s, colon cancer rates increased 1 percent to 2 percent per year between the mid-1980s and 2013, and their rectal cancer rates rose even faster, about 3 percent per year, the study found.

The disease is still very much linked to aging. One in 100,000 people in their 20s will be diagnosed compared with about 50 in 100,000 in their early 60s, according to the study.

But as Eicher discovered, the disparity in cancer incidence leads doctors to assume young people aren't vulnerable.

The disease has declined among older adults mostly because of widespread use of screening colonoscopy, which is recommended for people of average risk starting at age 50. Scoping the intestines can detect precancerous polyps that can be removed before they turn malignant. Although colonoscopy is considered the gold standard, less invasive, less costly tests are also available for screening.

The increase among younger adults is both ominous and puzzling. Experts say obesity, sedentary lifestyles, diabetes, and unhealthy diets are important factors, but unknown genetic and environmental changes must also be at play.

The human papillomavirus (HPV) is not believed to be one of them. It causes cervical cancer and some rare neck and genital cancers, including anal cancer, but only a small number of  rectal cancer cases.

To try to tease out other risk factors, researchers are studying genetic signatures and epidemiological characteristics of young colorectal cancer patients, Medscape,  a web resource for physicians and health professionals, recently reported. For example, early-onset colorectal cancer is more common in African-Americans and people with inflammatory bowel disease.

"Trends in young people are a bellwether for the future disease burden," said American Cancer Society epidemiologist Rebecca Siegel, who led the new study. "Educational campaigns are needed to alert clinicians and the general public about this increase to help reduce delays in diagnosis, which are so prevalent in young people."

When Eicher was 21, her doctor suggested scoping both her upper and lower digestive tracts, but her health insurer refused to cover the colonoscopy, deeming it medically unnecessary because of her age.

"My doctors didn't fight for me," she said. "They said the bleeding was probably hemorrhoids or a fissure [tear].  I figured, 'I'm not going to fight with my doctor.' You don't want to be the crazy patient. "

Eicher has a Change.org petition, signed by 81,000 supporters, urging that guideline groups change the  initial screening colonoscopy age to 20.

But she also advocates practical, low-tech, low-cost measures — things that experts endorse.

One is finding out your family medical history, because up to 15 percent of colon cancers have been linked to a genetic defect.  "Knowing your family history can put [colon cancer] on the radar for your doctors and allow you" to get coverage of a colonoscopy at a younger age, Eicher said.  (However, her own genetic tests came back negative for such a defect.)

You should also pay attention to changes in digestive or bowel habits such as diarrhea, constipation, pain, bleeding. Eicher uses her smart phone to keep notes and remember what to mention to her doctor.

"While I was pregnant with my son," she recalled, "I developed more symptoms such as excruciating pain with bowel movements, thinning of my stool because of the huge tumor, and also mucous.  I started having premature contractions at 30 weeks because of the tumor. They still never checked me, even after I complained of a growth I could feel. They assumed again that it was pregnancy-induced hemorrhoids."

"I'm a cautionary tale," she added. "If anything seems 'off,' it's always good to research it [online], and if you have concerns, then reach out to your doctor's office and say, 'Is this normal?' "

The American Cancer Society lists these signs and symptoms of colorectal cancer: