I agree with you that pain occurs after being treated unjustly. I think the sequence is as follows: 1) Someone is unfair to you; 2) Next comes shock or even denial; 3) Then comes pain, as you describe; 4) If the pain does not lessen or if you have no effective way of reducing and eliminating the pain, then you may become angry.
That anger can be at the person for acting unfairly, or at the situation, or even at the pain itself that resulted from the unfair treatment. It is the anger, if it abides and deepens, that can lead to health problems (fatigue, anxiety, and so forth). So, I emphasize anger within Forgiveness Therapy because it, in the form of excessive anger or resentment, can be dangerous to health, relationships, and communities.