“I’m not angry,” (said when clearly and visibly showing signs of anger.) Sometimes this is then followed with stonewalling. Often people say this as a form of denial, or they want to avoid conflict and not talk about what went wrong.
If you are angry about something, admit it and address it in a respectful and respectable way. First, get into a calmer state, think about what is going on for you, and plan how to bring it up with respect for both of you.