It looks like 頭を痛める is an expression meaning “to be concerned about”. Though like @xarde, I would have expected it to be ひとりで…
I’m a little confused by the opening two sentences of chapter 6.
その日、和子はちっとも授業に身が入らなかった。もっとも、どの授業も一度教わったことばかりだったのだが…。
First sentence: That day, Kazuko didn’t try very hard in class. Though, every lesson was only stuff she already learned once.
Is that pretty much right? I’m having so much trouble translating these sentences, particularly the second one. And I know that redundancy is much more acceptable in Japanese than in English, but having the sentence start with もっとも (尤も) and end with だが still feels so strange to me.
Finally, twice on the first page, it says 一日だけ. What meaning of だけ is this exactly? “Only one day” doesn’t fit well from the context because it implies she’s considering that time could have gone back more than one. Perhaps the word “merely” or “simply” is better for an English translation? Or is it something else entirely?
As far as story, just from the first page, Kazuko sure went quickly from “time reversed a day” to “time reversed for me a day”, and therefore from “I’m so confused, this makes no sense” to “ah, that’s the only logical explanation”. Just thought it was kind of funny.