@Richman I don't think they explained it,
BUT we can take that as a mystery to solve!
[ol]The last time we saw Himawari's tan was chapter 30. [/ol]
[ol]In her next appearance, chapter 32, it was gone. [/ol]
[ol]Back in chapter 24, we can see that various students have started wearing their winter uniforms. [/ol]
[ol]In chapter 33, Yuriko talks about it being cold, and in chapter 36, it snowed. [/ol]
I think we can infer that the weather was changing and that winter had come. Given this, and the fact that her tan is gone, we can assume that Himawari's athletic club activities moved to indoor training due to the change of seasons. That is my theory as a humble apprentice detective.