That's an edited page from the manga. Originally it's Kagami slapping Misao for asking if she's depressed because she gained weight.

Ayano probably would do that when she's angry.

I'm a beginner to Japanese, but I don't think there's anything to translate here. "Hi-hyaaa!" is just a scream, and "pashiin-" is just a sound effect, right?

If I were more certain I'd just remove the tag.

Correct, nothing meaningful here.