Actress opens up about fighting stage 3 breast cancer at age 19

Miranda McKeon, the 19-year-old star of "Anne With an E," said she felt like she was in her own movie when she was diagnosed with breast cancer this year.

McKeon was diagnosed with Stage 3 breast cancer after she felt an unusual lump in her breast.

"I think it's one of those movie-like moments in someone's life," McKeon said of her diagnosis in an interview with ABC News' Kaylee Hartung Tuesday on "Good Morning America." "I just didn't feel like it was real."

Before McKeon could begin chemotherapy to treat her cancer, she had to confront a potential complication of treatment, her fertility.

"It was explained to me that with chemotherapy treatment, there is risk of infertility down the line," she said. "And fertility was not something that I had ever considered."

McKeon said she went through the process of hormone injections and freezing her eggs, which she calls a "preventative safety net" in case she is not able to conceive naturally later on.

She then began chemotherapy and has now completed six rounds of the treatment.

"I am spending every other Tuesday in the clinic while my friends are at class," she said. "I'd rather be in class. I'd rather be doing a lot of different things. But this is my job right now."

McKeon has documented her cancer battle on Instagram, sharing her journey with her followers.

"I think I've put my head down and been like, 'You know what? We're going to keep this on a positive track because it could go dark quickly,'" she said. "But that's not where I need to be living, to hunker down and get through this."

Breast cancer is the second most common cancer among women in the United States, according to the Centers for Disease Control and Prevention.

Women younger than 40 account for just 4% of breast cancer cases in the U.S., according to the American Cancer Society.

Among women in the young adult and adolescent age group, though, breast cancer is the most common type of cancer diagnosis, according to Dr. Niki Patel, assistant professor in the division of medical oncology and therapeutics research at City of Hope, a cancer research and treatment center in Los Angeles.

"The diagnosis is a little bit different," Patel said of women who are younger. "We rely less actually on mammograms and more on ultrasounds and breast MRIs, and that's really because younger women tend to have more dense breasts and so you may not see as much of the detail on a mammogram, whereas you may get that detail on an MRI."

Patel, who is involved in McKeon's treatment, said younger women also tend to have more aggressive breast cancers.

"When we are working them up, we are more likely to perform additional tests such as CT scans, bone scans, PET scans to try to understand what their cancer is doing," she told ABC News. "Additionally, we're much more likely to order genetic testing ... aimed at diagnosing a mutation that may cause breast cancer."

The two BRCA genes (BRCA1 and BRCA2) normally help protect women from cancer, however, some women may have mutations to their BRCA genes, which can actually lead to cancer, according to the CDC. If untreated, women with a BRCA gene mutation are at least seven times more likely to get breast cancer before the age of 70, when compared with women without the gene mutations, according to the CDC.

Women are recommended to start getting an annual mammogram at age 40, or even earlier if they carry risks such as a family history of breast cancer, according to Dr. Eleonora Teplinsky, head of breast medical oncology at Valley Health System and a clinical assistant professor of medicine at the Icahn School of Medicine at Mount Sinai.

In addition to getting the recommended screenings, Teplinsky said women need to pay attention to their bodies and advocate for their own health.

"What I try to push across is that you know your body the best, and it is our job for all of us to make sure that we are getting the care that we need," she said, adding that means "knowing when something doesn't feel right to your body [and] educating yourself on what feels normal for you."

"If you feel a mass or you feel a lump, advocate for yourself to get the workup that you need, even if someone might say to you, 'I think this is an infection,'" Teplinsky added. "If you're worried and you don't think it's an infection, advocate for yourself."