When Govind Sandhu finished a half marathon in May, he felt surprised by how challenging it was. “I remember finishing the race going, ‘This felt just a little harder than expected,’” the 38-year-old from Sydney, Australia, tells TODAY.com.

“The next day, I woke up and I had a swollen knee.” He expected a puffy knee because he had recently fallen and injured it. But what occurred next felt puzzling.

“I started getting really wild flu-like symptoms out of nowhere. I couldn’t get warm. I had a really long hot shower,” he explains.

“I went to bed, and I had wild sweats the whole night where I was just drenched.” About a month later, Sandhu learned what caused his mysterious symptoms — he had stage 4 non-Hodgkin lymphoma. “Leading into my diagnosis, I’m the fittest and healthiest I’ve ever been,” he says.

“What I’ve learned through this whole process is that cancer really doesn’t discriminate.” Sandhu, a self-proclaimed “fitness fanatic,” enjoys CrossFit, runs marathons, and recently finished the . The weekend before the marathon, he was visiting Tasmania and trail running with friends when he had “a really big fall.

” “It was very bizarre for me to do that,” he says. “I got quite cut up and bruised up and had a bit of a swollen knee.” He rested before the half marathon but did not do as well as he expected on race day.

When his knee became swollen and he developed flu-like symptoms, he first wondered if he was rundown from s.