Korn does generate a certain emotional reaction, that's for sure lol. It's been a looooong time since I listened to a song from them, though. So Shu-chan has an injury that is causing consistent pain, sort of similar to the constant "pain" she's been feeling whenever Aya pops up. She's such a mysterious character; it's nice unraveling something new, while we continue developing Aya's troubles and worries.
The question of what Shu-chan actually feels reminds me of "My Dress Up Darling," when everyone wondered whether Akira liked Marin or hated her. That "mystery" was also fun and had its own unique conclusion. The personalities almost match up perfectly, too, with Akira being Shu and Marin being Aya, with their artistic bf/gf waiting on the side.
I think Aya spending some time away is fine for now. Sometimes you do need time to process and think about the things bothering you, and clearly, Mitsuki's avoidance and potential trauma about the band situation is shaking her a bit. In that way, "painkiller" could be referring to both the music joke but also what Aya's currently doing, hanging out with friends and thinking to herself. As always, I'm enjoying the piecing together of the bigger picture. Everything is mostly in place now, except for the ultimate role Shu-chan plays.