I find the answer in Volume 4, chapter 24 ( read from right to left, if you confused with this format, look at the page number )
Ishida realized that what Kawaii do to her appearance is clearly the same with Shouko at chapter 23. It's pretty clearly that Kawaii like Mikoshiba. So The reason why Kawaii change her appearance because she want to look attaractive in front of Mashiba. It's same with Shouko case. it's just my speculation but from the interpreter of the story at chapter 23 said that Shouko want to look attractive ( she want to looks different ) in front of Ishida, and after that he confession to Ishida but it didn't go so well.