The first time Lavina transformed was when she was five, her older brother, Draco, was six at the time, having had his birthday a few days before. She lay in bed as the sun glowed high at eight o'clock at night daydreaming about running around the Malfoy's luxurious garden at sunset. She was picturing herself as a tiny rodent that was small enough to squeeze through the cracks at the bottom of the old door that leads onto their garden. She daydreamed hard, as she had done every night for the last month or so when the sun stayed up until long after she had gone to bed, until she felt a bang on the inside of her head, like a force was trying to get in. She denied its entry until the banging became far more excruciating than normal, leaving her doubled over in her soft sheets, clutching her head and almost on the verge of screaming. As soon as she closed the mental barriers in her mind, the energy pushed itself through her body, leaving her in an uneasy state of calmness as she sat up straight. The energy tickled the inside of her body as they quickly began working on finding their way to the parts that were to be changed. She savoured the final ending of the pain in her head, but she didn't get the chance to for long. Like soldiers preparing to attack their enemy, as soon as the energy filled her body, the wave of pain doubled from the one she had in her head before. She felt as her torso caved in on itself, sucking the breath out of her as it shrank her lungs. She felt as her neck contorted into being smaller, pushing and shoving as it shrank, became longer then got larger again; she felt as though somebody was gagging her while they twisted her neck, she was expecting to feel it snap and to die. All at the same time, she felt as the features on her face mangled together to be pulled at, twisted and reshaped as though she was made out of clay. Her eyes felt as though she had lit a match straight onto them as her vision became more cloudy and her peripheral became larger as they strolled across to the side of her painfully elongated face. Her nose began stretching and twisting itself around to form a long blunt shape that stabbed at her skin as it transformed. She felt a burning sensation on the tip of her stinging nose and inside her ears and watched through streamy eyes as they formed a small ombre from their old colour to the new one, leaving her with a baby pink nose by the end of it. The tail came through at the exact same moment as her arms legs began shrinking and changing shape; her tail came through her backside like somebody was dragging her intestines out from her body, her joints being thrown around inside of her body, messing with her centre of gravity causing her to fall straight onto her face. Her paws came through as a mix between her toes and her ankles, pulling themselves in and out of shape and forcing her toenails to grow into tiny, sharp claws. Finally, she felt a thousand tiny prickles over her body as thin beige fur pushed itself out from her body, leaving her with the sensation of rubbing herself all over on a cactus.
All of this was accomplished in a matter of seconds, and once it was finally over, the invisible gag on her mouth that prevented her from screaming came free. And she screamed as though it was the last thing she was ever able to do again as she stared around at her horribly new, mangled figure, hoping against all of Merlin's odds that she was just in a horrible dream. She knew nobody could hear her as she heard herself and her barely audible mousey and underwhelming squeak. The piercing pain took over her body for roughly an hour as she lay there squeaking in her sorrows, vowing to never daydream about anything like this again.
Eventually, the pain across her body eased down to a manageable throbbing. If truth be told, she had no idea how long had passed; she didn't know how to undo this horrid hex she had put upon herself. She shook with the fear of her dear brother wandering into her room in the morning just to discover a pile of rodent droppings that would end up consuming her bed. She was mostly afraid of how she would walk. She was a human and the way humans walk is easy: we have two legs that bend at their knees, which allows them to move and stabilise themselves. However, mice have four legs, weak knee joints so rely on the strength of their ankles and use their tails to stabilise themselves as they walk and jump around. Lavina lost all hope of being able to live like this and just wished she could go back to being herself.
Of course, she thought this was impossible, but that didn't stop her from not trying to adapt to her surroundings. Instead, she lay in the tangled mess of her sheets that she had created from struggling against the magic that overpowered her. She spent the rest of her night laying like that, watching as the sun cast dancing shadows around her when it came time for it to go to bed.
As she had feared, once the sun re-rose after its nap from the sky, Draco raced into her room, flinging the door open and letting it bang on the wall. His footsteps stopped dead as he noticed the chaotic mess on his sister's bed. Slowly, he crept forward to the writhing, squeaking mass of his sister. He only saw a dirty mouse trapped where his sister ought to have been, begging for him not to hurt her with the heavy-looking broomsticks he held in his hands. He noticed the mouse's giant, begging purple eyes staring up at him, its body limp and its harsh breathing. If he were anybody but Draco Malfoy, he may have realised that it was his sister trapped in the body of a mouse, or he might've taken pity on the creature. But, as I said, this is the dearly beloved Draco Malfoy: too stupid for the first and too spiteful for the second.
Lavina only feared about Draco's worry for her not being in bed, but did not quite expect the sudden, violent outburst from her brother. He let out a long, loud scream, almost like a battlecry, as he brought down their broomsticks on her, nearly hitting her every time. Each time a strike hit her bed, Lavina bounced a little bit and squeaked in fear, begging him to stop, pleading with him to realise it was her. But he didn't. His anger began turning into Lavina's panic, which fuelled her into finally trying to gain control over her new limbs. First, she struggled with standing up and finding the best way to balance her body. Each time she fell down, Draco's broomsticks came with her, skimming the top of her nose, getting closer each swing to actually hitting her. She knew her new bones were more fragile than her human ones were, so if she let him land a blow onto her then she knew it could be fatal.
Finally, she found the best way to stand up after many close calls, and being constantly rethrown off balance by the tidal wave Draco caused with his attempted murder. She bent her legs a little at her knees, a tiny bit more at her ankles and straightened her tail from the base all the way to the tip. It shocked her at how hard it was to manoeuvre her tail with its constant urge to flicker and change shape. Once she had that part practiced, avoiding her many near-death experiences along the way, she tried taking a step forward only to immediately fall back down. This time, she landed in Draco's aim; she watched as the heavily bristled broomstick came down close to her face. Fear pulsed through her harder than her rapidly beating heart.
The old energy began racing with her panic. This time, she did not fight it, she allowed it to race through to her limbs. In the microsecond that she could have been killed, the energy pushed her limbs into action and she scampered away. The energy carried her into a hole in the wall before leaving her with the wooden laith layer behind the wall and a light dusting of plaster that had crumbled off over the years. Without pure instinct to keep her upright, Lavina collapsed to the ground, breathing heavily and feeling as though she was having a heart attack. She immediately lost all of her built up mouse-instinctual energy and, now safe, fell asleep to the sound of a scraping broom head being dragged along the floor and her brother's cursing of "that damned rat".