When someone's upset you, it can help to get your feelings out in the open to avoid dwelling on them or trying to suppress them.
But you should be careful how you go about it, because you may say something you'll later regret. Keep the following tips in mind:
If you feel angry, allow yourself a cooling off period so you can be clear headed when you talk to the other person.
Create a letter detailing your grievances, but don't send it. Simply expressing how you feel can bring a feeling of release.
Roleplay the event with a friend to find out how you could handle it when the time comes.
Don't dwelve too deeply into your emotions when you talk to the other person. This might be inappropriate in the workplace.