There is dispute among halachic authorities as to whether a hysterectomy renders a woman niddah. Since you are post-menopausal, and halachically considered a woman who does not menstruate, you may rely on the opinion that you do not need to immerse after the procedure. (Further, any spotting that occurs after a hysterectomy would not render a woman niddah since it is generally from the vaginal wall and is due to trauma.)

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