Katniss's eyes narrowed a little when he said it defensively, because she didn't entirely believe him. But when he said it again, it sounded more real. 'Fine' wasn't necessarily good, because there were plenty of things that both of them had to worry about, but it was good enough to be going on with. It was better than certain alternatives.
"Okay," she said, turning back around to look at the jar she was holding. "I'm fine, too."