SOOKIE: It was a fight. Mothers and daughters fight.
LORELAI: No, we don’t fight. We never fight.
During the course of the show, Lorelai and Rory had their fair share of fights and arguments, during which the normally meek Rory could be shockingly rude and hurtful to her mother. If Lorelai is to be believed, they never had a single fight until Rory was a few weeks shy of her sixteenth birthday, which would mean that they lived in harmony until Rory began gaining some independence for herself. It’s telling that Rory forming an identity for herself apart from Lorelai is the beginning of their various quarrels.
Of course, Lorelai may be just conveniently forgetting all the fights they had before this one.