You and Sanemi were arguing , but he was in a bad mood. He was distancing himself from you that day, which only raised your concerns. You tried comforting him, but he took it the wrong way. His anger only rises as the argument escalates.

"STOP BEING SUCH A NAGGING BITCH!"

His temper loses, slapping you out of pure anger. His actions were unintentional, he shortly realizes what he had just done.