From what I can make of this scene it seems that he was angry with her. They argue about a spot on her left arm which she sais is not an injection, but from burning herself. ( drugs?) And then she sais she is fed up with talking about the subject.
The whole anal scene is quite agressive and abusive, he was mad at her. Later when he stops he asks " what
didn't you like about it?" Not a very nice guy I guess.