Sakura doesn't understand the true question being asked of her, and having another affirm her identity as a "nobody" hurts in its own way. She isn't wrong, really, in that sense.

"Yeah, I guess," she answers, also incorrectly. "I'm someone who always ends up causing problems for others... Someone who doesn't really have any value." It hurts her again to describe herself in such a way, more than it's ever hurt her before— and that is likely thanks to the lingering images of her friends in her mind that she can't escape.

"At least I—" she stammers out as if her heart is overriding her self-control. "That's what I've always felt."