Awkwardly put on

Toriel was exiting her bathroom still quite sleepy, and goes to Frisks (he is 18) room to check on him. Not relizing she was still completely naked and wet she opens the door to see her son sleeping, softly smiling at his sleeping form she goes closer to kiss his forehead.

Remembering that she needed to get dressed ( somehow not realizing she was naked), she leaves his room and returns to her own. Going to her dresser had shown she was out of panties due to it being laundry day, decided that nobody would notice.

She grabbed her dress and clumsily puts it on and checking the mirror to see it was on properly, missing the fact the back of her dress was caught by her tail thus exposing her butt to any lucky onlooker.