Frisk was in the middle of sipping at her hot chocolate, her biggest worry at that point being not burning her tongue, when Papyrus' door slammed open. She jumped and the table rattled, the mug nearly knocked out of her grasp. Hot chocolate splashed out of it as she quickly put it down. 

"S-Sans?" She looked up to see his horrified expression, and for a moment she was scared. Was he locked in another nightmare? Was he confused and mixing up timelines? She wasn't who she had been before, and she desperately hoped that Sans knew that in that moment.

When Sans' horror deflated and he began to cry, Frisk's fear instantly vanished. Concern and heartbreak replaced it, and Frisk was just as quick as Papyrus in getting to the shorter skeleton's side.

For a moment, Frisk was just as helpless as Paps. She didn't want to accidentally upset Sans more than he already was, given her very image could have been what set him off in the first place. She looked up at Papyrus, considered his silent plea for help, then turned her attention back on Sans.  

"Sans...Sans, it's okay," she softly said as she wrapped her arms around him. "It was just a dream. Just a really bad dream." She held him close, though was mindful of any move he made in case her presence made him uncomfortable. Taking a deep breath, Frisk glanced at Papyrus, then bent her head to whisper softly in Sans' ear

"This isn't one of those timelines, I promise. Papyrus is safe. Everyone is safe. And it's going to stay like that." She hugged him a little tighter as her guilt washed over her again. She swallowed it as much as she could, however, knowing her full attention had to be on Sans.