Pretty sure this is Tithe: A Modern Faerie Tale (2002) by Holly Black, the first book in the Modern Faerie Tales Series.
Here's a summary which covers most of the main plot points.
As with Triss, Kaye Fierch is unaware of her status as a changeling at the beginning of the novel, although she is aware that she can see fairies. Kaye is a pixie and her true identity only becomes apparent when she saves the life of an elf knight, Roiben, becoming entangled in the complexities of fairy court life. Like Mackie, Kaye is allergic to iron unless she has been glamoured. She has magic of her own and is able to create her own glamour.
As with the previous novels, the fairy world is shown to be strange, amoral, and yet also scintillating to humans. However, Black’s narrative acknowledges to a far greater degree, the pleasure (laced with BDSM overtones) of losing oneself in fairy land – especially for dissatisfied teenagers. Kaye, like Mackie and Triss, must learn new rules of behaviour in order to navigate these new worlds, unlearning many human mores. In many ways, this parallels the way in which our understanding of human morality and behaviour is challenged as we enter the adult world.
In a later novel in the series, Kaye admits her identity to her human mother. Her mother reacts badly, turning her out of the house. Kaye promises to return her mother’s ‘real’ daughter. When she does so, her mother has had time to reassess her feelings and accepts both Kaye and her human daughter. The suggestion is that family is not blood but emotional connection. Kaye decides to split her time between the human world, where she has many close friends, and the fairy world, where she continues her romantic relationship with Roiben.
And here's an excerpt from the book itself, where the protagonist, Kaye, tells the elf knight, Roiben, to kiss her ass:
"That's my third question: What is your full name?" She didn't know what she had done, not really. She only knew that she was forcing him to do something he didn't want to do, and that suited her fine.
Roiben's eyes darkened with fury. "Rath Roiben Rye, much may the knowledge please you."
Her eyes narrowed. "It's a nice name."
"Your are too clever by half. Too clever for your own good, I think."
"Kiss my ass, Rath Roiben Rye."
He grabbed her by the arm before she even saw him move. She raised her hand to ward off the coming blow. He threw her forward. She shrieked. Her hand and knee connected hard with the stone floor. She looked up, half expecting to see the gleam of a sword, but instead he pulled her jeans down hard at the waistband and pressed his mouth against the exposed swell of her hip.
