The whiplash IS the point here. After all, this has only explored Aya's POV so far. The delivery wouldn't have been the same if we'd seen Mitsuki's entire week off school right off the bat.
Aya's intervention was clearly what affected Mitsuki more than the bullying itself.
Whether this is her coming to terms with the idea that Aya and her friends really accept her for who she is even after being mocked like that, and thus realizing she can just be herself 24/7; or if this is a facade she's putting up / a defense mechanism to trauma that's kicking in with the help of Aya's support... we are yet to see.
Keep in mind that dude put her love song, which is irrefutably dedicated to Aya, on blast. And Aya not only assuredly realized that, but immediately going ballistic for her, is HUGE for Mitsuki.