Every inch of Rainbow Dash's body ached. The scream of knotted muscles, cramped from hours spent bound in unfamiliar and uncomfortable positions, filled her with agony. Bruises, welts, and bites covered her, placed seemingly at random, bright stars in a constellation of pain. Some of them flared anew as she shifted, wounds rubbing against the bed on which she lay, but the exquisite suffering of sore limbs stretching quickly eclipsed that sensation. Dash briefly worried about getting blood on Fluttershy's sheets, but a part of her didn't care; she wanted to bleed, to leave a stain, a mark, something that would last, something to serve as a reminder of what she had suffered. She would have the bruises to remind her, the soreness deep within her, maybe a couple of light scars, if she were lucky. And, of course, the collar. Didn't Fluttershy also deserve something to remember her by? Rainbow Dash's lips curled into a smile. Even her jaw was sore. She sighed, utterly content; she hurt so much, and it felt so damn good.
The pegasus was startled from her reverie by the sound Fluttershy returning with the first aid kit she'd mentioned. The quick rhythm of hooves on stairs was accompanied, ever so faintly, by a stream of murmured apologies, "...so sorry Dash, I don't know what came over me...." She entered the room with a gasp and an, "Oh, my!" before continuing as she made her way to Rainbow Dash's side, "...I've never done this before and didn't expect it to be so...."
Fluttershy fell silent as she began to unpack the supplies, but as Dash felt hooves parting her coat to inspect her wounds, the murmuring began again, "...so...," the pegasus drew out her next word, seeming to relish it, "intoxicating. Oh, I hurt you so much, I'm sorry."
Rainbow Dash interrupted her with a hoarse whisper, "It's fine Fluttershy. Don't apologize."
"Oh, but I must! Look at this bruise, it's just huge," Fluttershy gushed as she gently nuzzled the indicated area, high on Dash's flank. The rainbow-maned mare felt warmth and dull pain radiate from where the soft snout touched her abused flesh. She pratically melted into the bed, murmuring incoherently at the pleasant sensation. "Oh, this is going to hurt, Rainbow Dash, but it has to be done. I'm sorry." Dash barely had time to process that before she smelled the rubbing alcohol. One after another, stinging explosions washed over her as Fluttershy sterilized the few injuries that had broken the skin. Those were bites, mostly; Dash fondly remembered Fluttershy casting away the crop and taking things into her own hooves. And teeth.
The ponies remained silent, lost in thought as Fluttershy applied the small, pink adhesive bandages that the unicorn medical ponies mass-produced. Rainbow Dash, ever the daredevil, was more than familiar with them, though she'd never needed quite so many in one sitting. The yellow pegasus broke the silence with a whisper, "Turn over."
Rainbow Dash tucked her wings and rolled onto her back. She winced as sore muscles and battered flesh protested. Dash looked up at her caretaker, at the soft concern in eyes that just an hour before had been so mercilessly hard. Their gazes met, and for a few seconds neither moved. After a moment, Fluttershy leaned over and kissed her, just a quick peck on the lips before the pink-maned pegasus began studiously attending to the wounds on her underside. Rainbow Dash couldn't help but blush at the display of affection.
Eventually, Fluttershy finished her work. She crawled into bed next to Dash, who pulled her into a close embrace, heedless of the pain it caused. Face to face, bodies entwined, the pegasi enjoyed shared warmth and reflection.
"I'm sorry I hurt you, Rainbow Dash." Fluttershy said.
Her voice still hoarse from screaming, from pleading, the other pegasus could manage only a whisper, "I told you, Fluttershy. Don't be sorry. It was my idea. I asked you for it. I begged you for it."
"I'm glad you liked it," the yellow pegasus smiled.
"Fluttershy...," Rainbow Dash averted her eyes, hesitating. Her cheeks grew redder, "...Um, can this... can you not mention this to the others? I mean... you know my reputation. I'm supposed to be the cool one, the one in control all the time. The winner, the one on top. And out there, I am. But in here, with you, I can't help but want to surrender, to give in. To be beaten, in every sense of the word. And I really don't want everypony else to know it. So please don't tell them?"
Fluttershy's lips brushed across Dash's forehead as she whispered, "It can be our little secret, wings of my heart."
Rainbow Dash blinked, "Hmm? What'd you call me?"
The other pony squeaked, looking away, "Nothing."
Dash smiled, "Was it 'wings of my heart'?"
Barely audible, Fluttershy replied, "Maybe."
"You're adorable, Fluttershy." Rainbow Dash paused, "How did it feel? To be in control like that? To... to do the things you did to me?"
"I... at first I thought I would have hated it. When you suggested it, I just didn't think I'd be capable. But when I tried, really tried, the... the savagery of it," she sounded awed at the recollection, "The sounds you made. The jolt in my teeth whenever the crop connected. The way you never stopped whispering, begging for more and harder," She stopped, blushing, finally aware of what she'd been saying. After a pause, she continued, "It was... good."
Rainbow Dash smiled, "So that means we can do it again?"
Fluttershy grinned, her mischievous smile reminding Dash of their earlier play, "Anytime you want, Rainbow Dash."
The blue pegasus took a deep breath, pain flaring in bruised ribs, and when her lungs were full, she exhaled a single, quiet, "Yay...."