Not all staining will render a woman niddah. As long as you do not experience an actual flow of blood (comparable to a period) you may take precautions against becoming niddah from this discharge by wearing colored underwear or disposable pantyliners and waiting at least a few seconds (ideally 15 seconds) after urinating before wiping. This is true even if the discharge is a monthly occurrence. It is also important to note that not all shades of brown are problematic.
See our article on stains for more details.
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.