An Army veteran thought he knew the cause of his knee pain. Doctors found a rare cancer

While U.S. Army Staff Sgt. Rayshaun Smith served a tour of duty in South Korea in 2018, he started feeling mild knee pain. 

At first, he thought it was just from his rigorous military lifestyle or his intense workout regimen. He and his fellow soldiers were spending lots of time in mountainous areas, he said, and he thought that he might have done something to injure it. 

"I thought it was just everyday pain," Smith, 32, told CBS News. "It wasn't like it never happened before."