It was then the six-time Olympic gold medalist realized just how difficult the road ahead would be in her return to track. It was also when she became even more determined to line up for another sort of race: Campaigning for greater maternity protection from sponsors.
She along with other track moms spoke up. Their voices are starting to be heard. Case in point: Felix agreed to a sponsorship deal Wednesday with the women's apparel company Athleta that offers maternity safeguards and guarantees.
"I totally want to be remembered for having some impact on the sport, having changed things for women of the next generation to not have to feel like they have to choose between a career and motherhood ," Felix said in a phone interview. "If I could walk away from the sport with some of these issues looking a little different, that's a win for me."
Felix was only 32 weeks into her pregnancy when her daughter, Camryn, was delivered on Nov. 28 via emergency C-section after tests showed the baby's heart rate had dropped to dangerous levels. Camryn weighed in at 3 pounds, 7 ounces.
At the time, Felix was without a contract. She'd been trying to renew her deal with Nike since it ran out in December 2017. She wanted to start a family in '18 but was worried about the ramifications.
The 33-year-old Felix explained her situation in a New York Times editorial piece on May 22: Nike wanted to pay her 70% less. Although she was willing to accept a reduction, she wanted assurances around maternity. It was declined.
Alysia Montano and Kara Goucher also spoke out about the need for sponsors to support female competitors before, during and after pregnancy — that contracts shouldn't penalize someone for starting a family.
In a May 17 statement, Nike wrote: "Last year we standardized our approach across all sports to support our female athletes during pregnancy, but we recognize we can go even further. ... We recognize we can do more and that there is an important opportunity for the sports industry to evolve to support female athletes."
For Felix, taking an initial stand was daunting. She didn't know if there would be repercussions.
"I just felt compelled," she said. "I think it was becoming a mother and knowing that this is a world that my daughter will grow up in and even though it's uncomfortable and it's still scary, sometimes you just have to talk about your experiences. When I did I was just overwhelmed with the stories that women shared with me. It just reinforced that it was the right thing to do."
Athleta brought Felix on board with a wide-ranging partnership deal as she becomes the company's first sponsored athlete. The partnership includes such things as collaboration on initiatives to empower women and a hand in designing products.
"When you're supported in that way, you are now a better athlete, you're a better mother and you're a better person," said Felix, who's still working on obtaining a shoe contract. "It's already hard enough to be a new mother. It's extremely difficult to be an athlete. It's so much better this way."
The road back to sprinting hasn't been easy. Felix wasn't sure if she would even make it back.
She remembered one day in early December being in the newborn intensive care unit with her daughter, who was struggling to breathe.
"I just remember that day just feeling heavy and feeling like I don't know if I will return," she said . "I don't know if it's still as important to me."
Her first workout was a simple walk (it was taxing). Her first track practice wasn't until around March.
Gradually, Felix is rounding back into form.
At the U.S. championships last weekend, she finished sixth in the 400-meter final during her first competition in 13 months. A solid step forward and one that earned her a chance to possibly run in the relay at the world championships in Doha, Qatar, this fall.
She's on the fence about making the trip if selected. She wants to dedicate her time to training. Her priority is being in top shape a year from now for the Tokyo Games.
Plus, there are plenty of things to do at home — like take her daughter to swim classes. She's getting the hang of balancing track with motherhood .
"The biggest change has just been not getting as much sleep and feeling exhausted," she said with a laugh. "But you figure it out and you just find a way."
Because she wants more moments like this: Her husband handing Camryn to her after finishing the 400 at nationals. Felix is yearning for a similar scene in Tokyo as she tries to make her fifth Olympic team.
"I look forward to hopefully being able to compete there and have her there with me," Felix said. "That's the biggest goal right now."