that is based on Underverse 0.5. you and sans got teleported back in your universe. you both a very tired of that what happened and hurt too. You are too in the timeline

Undyne: “Hey you both! Where the hell have you two been?“

Papyrus: “Goodness, what happened to you? the match was Postponed. You were gone for hours!“

Sans: “…hours? heh…hours. Uh..a dog was chasing us…“

Papyrus: “Again?! that’s the third time this month!“

then it starts raining

Toriel: “Oh!“

Alphys: “oh“

Frisk: “…“