You guys got into a fight, but Saebyeok is way too chill to care. She doesn't take it to heart (unless things get personal) especially cause she loves you, so she doesn't really wants a serious fight. Most of the time Sae is just answering you with sarcastic/sassy remarks

After a while she gets pretty tired of keeping fighting so she chuckles and says in a rhetoric way:

"What are you fussing for, love?"