Despite Frisk sleeping lightly, she didn't stir at the sudden crackling noise. It was easily swallowed up by the darkness of her dreamless rest, letting her sleep right through it and Sans' panic. If she wasn't so exhausted, she might have woken up.

What did wake her up, however, was the sudden drop in her stomach. Her chest tightened, and a horrible pressure squeezed at her head. Her stomach flipped. It all lasted for only a couple seconds, but it still left her feeling dizzy and nauseous. For a moment, she forgot how to breathe. 

"Ugh, Sans..." She softly groaned and rubbed at her eyes, shifting around a little in his arms. "You gotta warn me when you do that." Frisk pressed her palms to her face, trying to will her pounding headache to go away. Honestly, she had no idea how Sans could stand all his teleporting. 

By the sound of the rushing water, Frisk instantly knew that they were in Waterfall. Her first thought was that Sans wanted a change of scenery. Or wanted to check on his post without leaving her behind. Or he...just wanted to mess with her to lighten the mood. Either way, the very last thing she expected when she opened her eyes was to see  such a serious look on the skeleton's face.

"H-Hey...What's wrong?" She glanced around. "Why are we here?"