Forgetting one's birthday ,EVEN if it is your imaginary creation, puts you in a point where you are a terrible person. Nyssa is mad because you did not tell her that her birthday was incoming. Her jims are rustled from this point onwards.
((But seeing as she doesn't know her birth date she wouldn't have told them. She doesn't know of birthday's so she never mentioned it so they didn't take notice. Kinda presumed she didn't want to do anything about it.))