Why does page 48 of chapter 11 where he pats his head has "stab" as a sfx? （￣へ￣）

It's not a fx for patting his head. I'm pretty sure the stab here was for Sihoo's emotions. As in "stabbing pain in his heart" sort of thing, because he was thinking about how he doesn't want to forget again.

I am quite sure that the stab sfx was referring to the inside of his head, resulting in having him the intense dreams at the end of the chapter. Don't forget that his memories come back from time to time. Afterwards he was feeling dizzy, so they went to bed afterwards.