I know how it feels , one negative feedback - looses your sleep , but calm down take deep breaths , think about why was this ? But before that I would definetely contact the buyer and ask for the reason and if I feel , say sorry.this will not do any harm but the buyer feels nice and calms down. asking for a refund? give it immediately ! that what I did, even though I knew evrything was ok. It was ur bad day , thats it !
sometimes people don't even see the feedback later, keep doing good work.