Stains on toilet paper may be disregarded as long as at least a few seconds elapsed after urinating before wiping.
However, even if the stains found on toilet paper did not render you niddah, bright red stains found on a tampon that was inserted internally likely rendered you niddah, even if you can attribute the bleeding to uterine polyps (although it is not clear that this is necessarily the case). There is halachic dispute whether bleeding caused by uterine polyps renders a woman niddah. Some poskim consider it dam makkah (bleeding from a wound, which does not render a woman niddah), while other poskim consider it dam niddah.
Furthermore, any bleeding that occurs more than a year after menopause should be checked by a physician. Therefore, we suggest that you make an appointment with your physician. At that time you can ask if s/he feels that the bleeding is due to the polyp or other reasons. If there is a reason found that involved only the cervix or the vaginal canal, please get back to us.
In the meantime you should assume you were rendered niddah, and you may begin counting the seven clean days if the bleeding has stopped and more than five days have passed since the bleeding began.
Please feel free to get back to us with any further questions.
B’Hatzlacha!
This response has been updated to reflect the rulings of our current Rabbinic Supervisor, Rav Kenneth Auman, regarding waiting before wiping.