It does not help that Historia gets turned on by Ymir being a flustered fuck. Usually, Ymir is obnoxiously cocky (which Historia will never admit is pretty hot too) so when that smirk is wiped off her face Historia will try to use it to her advantage.
They're making out in Historia's bedroom. Passionately, hungrily. The kiss is heated, and Historia starts kissing her more roughly, her way of letting Ymir she wants to go further. Historia lets out a soft moan into the kiss, she's getting desperate.
Sitting on the edge of her bed, Ymir breaks the kiss, smirking at her girlfriend blushing. She looks into her eyes- but then something shifts. There's this look in Historia's eyes, something so devious there's no way it can possibly belong to her.
It's Ymir's turn to turn red as Historia, perfectly composed, unbuttons her shirt at a ridiculously slow pace. It's as if she's taking her time on purpose, and Ymir can't draw her eyes away, her mouth is dry. Historia is shirtless now, and Ymir just sits there like an idiot, staring. She can't help it, Historia happens to have amazing boobs and the bra is pretty revealing. Ymir feels herself getting dizzy, lost in awe at the sight of her girlfriend's chest.
Ymir wakes up on the bed two minutes later, completely disoriented.
"You okay, babe?" Historia innocently asks.
"What happened?"
"You passed out, and your nose is bleeding."
Ymir tries to wipe some of it off.
"Holy shit." she sits up.
Historia huffs. "Ymir." she whines.
"What?"
"...This is the third time this week."
"I can't help it babe, you're really fucking hot."