It is possible to become niddah from uterine breakthrough bleeding caused by taking hormonal medication irregularly, despite having had an oophorectomy.
However, if the bleeding was light enough that you didn’t need a pad, then it is considered staining and is subject to the leniencies of ketamim. Thus, stains found on colored underwear or on disposable pantyliners may be disregarded. Stains found on toilet paper may be disregarded as long as at least a few seconds elapsed after urinating before wiping. Stains found on your skin or on white garments depend on the color and size of the stains. See our articles on stains and toilet paper for more details and get back to us if you are still unsure of your status.
If the bleeding continues we recommend consulting with your physician.
B’hatzlacha!
This response has been updated to reflect the rulings of our current Rabbinic Supervisor, Rav Kenneth Auman, regarding waiting before wiping.