Shatter is up to her weird evening antics and is wearing socks to represent this. But at the same time, seems to be engaging herself into fetish territory and that one was the omorashi one. Basically, she has been purposely holding her pee so that she get the erotic feeling when about to burst. This takes her a long time to get to that point, but it doesn't stop her from messing with the other ponies and that they needed to hurry up in the bathroom, only for her to disappear when a pony comes out. Despite the supposed teasing behavior, she is uncomfortable physically and wants to empty but tells herself no.

Only time will tell before her bladder gives out completely, and she probably will be waiting in line for the bathroom for real, she definitely won't be smiling when she's close to pissing on the carpet. The other ponies will probably be mad at her for forcing them to take care of business only for her to not be there after the whole mess is over.

And no, the door doesn't actually have the "WC" (Water Closet) on it, I placed it there so that people would know that the door that my character is at is that room. And Water Closet is a room with just a toilet, although this here is in the wrong context, so it's probably a bathroom, with a bath tub/shower or a bath tub, a shower stall, sink, and toilet.