Stephanie Wysaski was diagnosed with a cancer that can be hard to detect.

Earlier this year, Stephanie Wysaski found out she was pregnant and expecting her fourth child.

What she didn't expect was a breast cancer diagnosis.

"It was a shock. It changed the course of my pregnancy, all of it," Wysaski told "Good Morning America."

So while the 35-year-old has been nurturing a new life for the past several months, she's also been fighting for her own life.

"Whoever thinks about getting a cancer diagnosis while pregnant?" Wysaski said.

Weeks after learning she was pregnant, Wysaski said she watched a TV segment about performing breast self exams, and that prompted her to do one herself.

"I feel a lump, a pretty sizable lump high on my chest," Wysaski recalled. "But I was nursing my then 11-month-old. I was newly pregnant. Things get lumpy when you're nursing."

Wysaski's doctors performed tests and confirmed she had invasive lobular carcinoma, a type of breast cancer that is harder to detect on mammograms and accounts for 10%-15% of breast cancers.

"It was growing pretty rapidly, and the fact that it moved to a lymph node, we're dealing with something that we really need to address as quickly as we possibly can," Dr. Annabelle Veerapaneni, a Northwestern Medicine oncologist, told "GMA."

When Wysaski's baby reached 15 weeks, doctors at Northwestern Oak Brook prescribed Wysaski a chemotherapy regimen that both mother and baby could tolerate.

"The mainstay of treatment that is given during pregnancy is very, very safe," Veerapaneni said.

Wysaski's chemo treatment ended in August. Speaking with "GMA," she said it has been a whirlwind experience.

"As much as I'm in my own head thinking, 'Cancer, cancer, cancer,' and spiraling, my kids still have school pickup and drop-off, and they still want cuddle time," Wysaski said. "I just didn't have time to worry. Life doesn't stop."

Wysaski is due to deliver her baby early on Tuesday, at 34 weeks. She said she's looking forward to meeting her new addition.

"She's kicking, rolling, punching," Wysaski said. "It's almost like a good sign. How I take it, it's just that she's strong in there. She's tough. She's letting me know that she's all good."

Mom diagnosed with breast cancer while pregnant with 4th child