Hi Joseph,
As Walt mentioned, your anger could very well be tied to the imminent visit of your mother, someone with whom you have very negative associations. While I think forgiveness can be healthy, I also think it is important for you to honor and experience the feelings of anger and resentment that you may have towards her for contributing to what you termed a "pretty rough childhood". Now I'm not suggesting that when your mother arrives you should yell and scream at her. But I think that it is important for you to let some of theses feelings through, feelings that perhaps you have not given yourself permission to feel until recently. Where has the anger you have been experiencing been directed? What has triggered it? Have you felt anger and rage before when thinking about what you were forced to cope with as a child? By feeling obligation to suppress and contain your emotions you may be tapping in to previous patterns that have you downplaying and ignoring your needs in order to "keep the peace". As these patterns can be associated with periods of suffering and anger, they can bring these emotions to the surface.
It may be beneficial for you to take some quiet time to yourself before your mother arrives and allow yourself to feel, experience, process and move through the anger you are feeling. Reflect on your triggers and look at the things you are journaling about and perhaps you can understand where this anger is coming from. As you understand it, legitimize and honor it, you will find it easier to move to a place of peace with your mother's presence, as she no longer has the power to diminish you or make you suffer.