Fluttershy was helping out Pinkie Pie for a birthday party for the cake twins at Sugarcube Corner. She was filling up balloons while Pinkie worked on the baking. Because Fluttershy was a weak blower, she used the helium tanks that Pinkie always had in stock to blow up the balloons and tie them.
 
She completed about two dozen balloons but still had quite a few left to go through, she definitely had enough helium to fill the whole room with balloons.
 
Suddenly the twins came running down the stairs, Pound cake speeding over the stairs in full flight, and pumpkin cake floating over everything in her magic. They zoom pass Fluttershy, leaving a gust of wind to blow her mane in her face.
 
“Oh, kids!” Fluttershy says in a weak shout.
 
She stops her balloons and takes off after the twins. “You shouldn’t be running around the house. You could get hurt!”
 
Fluttershy chases them into each room in the store, passing the kitchen where Pinkie pays no attention to the racing horses. They make their way back to the front of the store, the twins take a sharp turn for the stairs and bump into the helium tank. It wobbles back and forth, while the twins take a U-turn from the stairs and pass over Fluttershy’s head. She tries to turn quick but slips up and lands face first into the floor, she stands up from her back legs first when—
 
“Gahh!” She squeals as something lodges itself into her rectum. Blushing hard, she turns to see the tank spout slipped between her butter cheeks. A rush of air blasts through her anal cavity, sending a chill up Fluttershy’s spine.
 
“O-oh my.” Fluttershy squirms awkwardly to try and un-lodge the spout, but it seem to be in pretty tight. “Oph!” Fluttershy winces as she feels air pressure start to fill up around her flanks.
 
First they tingle, then they begin to stretch out. She watched in horror as her cutie marks pull apart and are enlarged; as they grew into basketball sized spheres they start making balloon like stretching sounds.
 
“Oh dear,” Fluttershy whimpered. She tries desperately to kick the tank away from her plot, but by then her cheeks were firmly squashed tight around the head of the tank. She could feel air now seeping through her belly, filling it up with helium.
 
Fluttershy starts to waddle across the room,— not getting far, as her flanks inflate and cover more of her back legs, rendering them almost useless. Sweat drips down Fluttershy’s temples. She looks wide eyed at the twins that happened to stop their racing to stare curiously at Fluttershy’s wide hips.
 
“Oh, foals! G-get Pinkie! PINKIEEEEE!” Fluttershy covers her mouth as her voice sounded a few notes higher than she usually does.
 
The twins start snickering and giggling like the foals they were, not moving an inch to do what she told them to do.
 
Fluttershy feels a new pressure emerge in her gut, as she looks down her pupils shrink at the sight of her fast bloating stomach. It grows out and expands to about the size of a five month pregnant mare. She finally realizes it’s because she held mouth shut and lets out a breath of air, but her stomach only shrunk half its size, and was also regaining lost air from the helium tank. At this point Fluttershy was screaming Pinkie’s name in a chipmunk voice while her flanks grew pass beach ball size.
 
Pinkie was shaking a large bottle of soda when she hears the high pitched noise in the main room. Without thinking, she drops the bottle on its side, when all the pressure inside finally bursts the cork off and sends it flying into the main entrance. The cork bounces off the walls, ceiling and floor, then rebounds off the wall and passes Fluttershy’s gaping mouth.
 
“Hemph!” The cork lodged itself passed her jaws and blocked the airway and silencing her for good.
 
*Crrrreeeeeeeeaaak*
 
Fluttershy’s belly blimps with all the helium now storing itself primarily in her belly.
 
“Mmeph, MMMMMPHMPH!!” Fluttershy cries with the cork cutting off her voice. She begins to lift off the ground as her tummy reaches the size of an exercise ball, catching up to her fast with her impressively massive flanks that had her back legs incapacitated a minute or two ago as they were mostly consumed inside the round spheres.
 
As she floats higher, her chest also starts to push out with helium, her legs being spread out with the tight round belly combining itself with her chest, and soon it creeps up her neck. Fluttershy felt tight, too tight for that matter, like how a balloon feels when it reaches… oh dear.
 
But as soon as she hit the ceiling and lightly bounces off, she feels the spout of the tank slowly slips out of rectum and passes her wide cheeks to fall to the ground with a bang.
 
Pinkie comes out and looks at the giant yellow blimp mare, Fluttershy’s belly stretched out to fill up half the space in the room, while her flanks press into the ceiling and left only a quarter of space between her plot and the ground. Her head was almost invisible being consumed in her helium filled body, her expression looking helpless and frightened.
 
“Silly Fluttershy, you’re suppose to fill the balloons with air, not you!” Pinkie says without a hint of worry.
 
Pound cake flies up to the middle of Fluttershy’s tight chest and gives her a good push, which was followed by a loud creak from her skin. She starts to lightly float in the opposite direction while her body tilts sideways. Pumpkin cake, cheering and laughing, joins her brother in the air using her magic again to fly to the other side where Fluttershy was floating towards, then she gives her large plot a push back to her twin brother.
 
Pinkie snickers and snorts, while Fluttershy whimpers and moans as she’s tossed back and forth by the foals like some toy ball.