Disclaimer: I don't own Undertale, Toby Fox does. I just wrote this story.
No sooner had the words passed her lips had she abruptly stood up and booked it up the stairs, flying into her room and falling back against the door-frame. Her heart broke at the thought of them not being together, she loved him, had been for quite some time, and last night replayed heavily on her mind. She took in a shuddering gasp before bringing her hand up to her mouth. Biting roughly against her fist she sobbed quietly, letting herself let out all of her pent up emotions.
How could sans want her? How could he love her? After all, anytime he was "with" a woman, she was discarded the next day, as if they meant nothing to him. And although she had professed her love for him, her mind wouldn't stop running a mile a minute with doubts.
Her heart screamed at her to be by his side for however long he would allow it, but her mind told her not to be so foolish, that he could never have loved her and to quit while she was still ahead. She knew he had some ounce of care for her since he was so gentle with her last night, but she wondered if he only pitied her weak attempts to be something more with him. She wondered if he only saw a desperate little girl and had only taken such care with her so as not to hurt her during her first time.
Her heart shattered, silently agreeing with her mind. How foolish she was to believe he could really care for her.
Just as quickly as that thought came, she heard a soft knock on the other side of her door.
Sans could only stare at her in shock, not even noticing that she had fled from the table at break neck speed. His thoughts scrambled and his breathing stopped. Just why would she want to end their... relationship? Whatever it was that was growing between them, he had enjoyed it. After growing to care for her and eventually realizing that he had deep feelings for her, he had somehow messed it up. Hadn't he?
With a dark chuckle and quickly filling eye-sockets, he wondered if last night had been a mistake? After all, it was just sex to him, but to her... he was her first. He took away something so precious to her, something that could never be regained. He felt the tears roll down his cheeks, his entire body shaking lightly. Filled with shame and guilt, he couldn't help but wonder if she was regretting last night.
'Of course she would' he thought bitterly. 'I had to lose control and take her, even though she wanted it at the time, now she must realize how precious her virginity was. Wasted on me.' He couldn't help but laugh self-deprecatingly. He stood from the table and snapped his fingers, warping into his room. He faintly heard her sobs and although he wanted to go to her, he had done enough damage.
He threw on the first shirt he could find and warped out into the chilling air of Snowdin.