One never could be sure of how one's quirk could be affected by a simple cold, that was a fact that every hero know and had drilled into their head over and over again..every hero that was save for Mount Lady. In her defense she would later on reason that who else was gonna try and help get those people down from the skyscraper they were trapped on now but the end result was all the same:
One second she was saving to the people she had just saved and turning to go and leave and the next with a mighty sneeze she had gone from the tallest being alive, to 3'4, though thankfully her costume had shrunk with her.
"uh..Ok..THAT'S new!" she squeaked out, rushing over to a window to get a look at herself groaning with despair as not only was she the height of a preschooler, she was as flat as one too!
"My lumps! my lovely lady lumps!" She cried out, her very voice small and girlish and not helping her over all image.
Before Yu could whine more she found herself being scooped up by a young woman who ironically towered over the little shorty.
"And what are you doing out here missy? besides giving everyone a panic attack?" the woman scolded. 
Yu was confused till she say the name tag on the woman's green apron, her name was Chloe and apparently she worked at a local daycare. 
"Uh, Ok listen, I know I look like a little girl but I'm really Mount lady!" Yu said quickly, embarrassed at the ease she was being held. 
"Mmmhmmm..Sure you are. just because we're having hero day at the daycare doesn't mean you get to race out and go and patrol the streets missy." The worker said, already heading back. 
"W-wait I can prove it! watch me grow!" The hero cried out and tried to use her quirk. instead of turning giant and towering over the well meaning worker all that happened was a thunderous and wet fart came out of her backside, staining the rear of her tights. 
"Gee, that really showed me. maybe next year you should go as Great Explosion Murder God Dynamight." Chloe said, wrinkling her nose in disgust.
"This can't end well.." Yu whined.

As it would turn out, Yu called it right as not only was she rushed in and checked into the daycare program, but she found herself over powered, stripped of her costume and put in a light pink t-shirt and a massive diaper after spending 10 minutes on the potty.
"Anyone who lets one rip like that has something epic brewing in them, and I'm not letting you destroy a pair of shorts." Was the logic that Chloe had given her.
waddling around the daycare, it was kinda surreal for Yu who was looking at at least 2 dozen little kids, all dressed up like other heroes that she knew and trained with. Of course with her in a top and a diaper she was the target of ridicule, being asked if she was super diaper girl or the like and anytime she tried to argue she was the real deal and go and use her quirk.. well..
"YUCK! ferget super diaper girl! your da pampered bomber!" one little boy said, holding his nose and running off.
"And thus, my legacy only grows." Yu grumbled, looking around and trying to spot a phone.
of course being seen like this by her friends was the last thing she wanted but if it was having Momo or Midoriya come down and pick her up vs spending the day in baby jail..well she could handle a bit of humiliation. and it wasn't like this was doing her ego any good anyways.
Waddling over to Chloe (and letting out a fart every step of the way even though she wasn't even trying to grow anymore) Yu tugged on her pant leg.
"Oh good, super stinker is here." The unpleasant woman said and looked down, holding her nose. "Let me guess, your pooping yourself?"
"No!" Yu huffed, then a super loud and wet fart ripped out of her backside, the diaper barely acting as a silencer. "Er.. One sec.." and she reached back and patted her rump, confirming it was much free for the moment. "No! Look, can I use the phone, I can CALL someone down here who'll be able to confirm my st-"
Yu was cut off as anther massive fart ripped out of her, the smell filling the room and she felt a little dizzy. 
"I think we better get you to a potty, double time." Chloe said, looking a little green.
"No! I don't need a potty!" Anther massive fart. I need to use a phone and prove that I'm.." even more poots. "I'm.." a series of massive ones, the diaper puffing out almost from the force of the winds she was making. "I'm a superhero!" Yu cried out.
before more could be said a final loud nasty fart sounded, and Yu felt herself growing taller even as her buns got all warm.  As she quickly reached eye level with Chloe she cried out in triumph.
"Ha! what did I tell you! I told you I'm a super hero! I am Mount Lady!" She giggled, then realized that her voice still sounded the same. 
looking down she would find out that she hadn't exactly grown like she's thought, and that the only reason her and Chloe were now eye level was she had unloaded a massive load in the seat of her diapers that had SOMEHOW managed to keep from leaking even as they expanded, her legs dangling in the air when not resting on the soft warm and smelly bean bag she had made. 
"...Ok two things: One, I DO believe that you've got a quirk..because THAT'S no natural." Chloe snickered. "And two..they don't pay me enough to change something like that. your on your own kiddo."
Chloe walked off, taking off her apron and tossing it into a trash can as Yu was left wiggling around, kicking her legs and wondering just HOW was she gonna get out of this one. 
which of course is a story for anther time.

the end