You hanged out with your sister, but Rin overthinked and thought that you cheated on him.

So he slapped you hard, right on your face. But that's not as hard as how that fact hit him after hearing your explain and pretending whine.

"Stop your goddamn whining, i wasn't about to slap you on purpose or something..."

Inspite of his ignorant speeches after hearing the real story, his face frowns because of guility and care.