There are two primary halachic issues that arise with gynecological procedures: (1) opening the uterus beyond a certain minimum size will render a woman niddah even if no visible bleeding is observed and (2) bleeding from the procedure may render the woman niddah.
(1) The question of opening the uterus depends on the size of instruments used. The instruments used in a standard hysteroscopy for polyp removal are well under the 19mm diameter used as a criterion by our site's halachic supervisor, Rav Yehuda Henkin. You should check with your doctor in advance to confirm what procedure you will be undergoing and the size of the instruments s/he will use.
(2) Given that the procedure involves direct removal of a polyp, you can expect bleeding for a number of days, followed by spotting. Halachic authorities differ in their approach to this bleeding. Some maintain that with any uterine bleeding there is concern that hormonal changes are involved, and thus consider the woman niddah. However, we follow the opinion of Rav Henkin that bleeding from polyp removal is a clear instance of dam makkah, bleeding from trauma, which does not render a woman niddah. This is especially true for a post-menopausal woman who does not experience regular menstrual bleeding. You should abstain from intercourse until the bleeding subsides but there are no other restrictions and there is no need to immerse afterwards.
Please feel free to get back to us with any further questions.
Refuah shleimah!