Laura gulps, there's no where she can stop to pee without risking discovery. She'll just have to make it to the shop. She's still an adult inside and won't let childish limitations beat her.
Her logical conclusion does nothing to convince her bladder. With each passing second she feels the pressure grow. Her pace gradually slows to a waddle as she struggles hold it. Her efforts are in vain and Laura' cheeks grow red as she feels the urine enter her underwear.
As a small mercy, her analysis of the underwear was correct. The abosorbant material does prevent the urine from soaking her clothes. The wetness against her crotch and butt does not provide much comfort though.