If the stain was found externally (i.e., not on a bedikah), and was smaller than a gris (the size of a US dime or Israeli shekel), it may be disregarded and did not invalidate your clean days.

If it was found on a bedikah, or if it was larger than a gris and found on a white fabric, it may have invalidated the clean days depending on the shade of brown. Light browns (the color of coffee with milk) with no reddish hue may be disregarded. Any other shade of brown needs to be evaluated by a halachic authority. The fact that it was likely old blood and that you are on the pill does not allow you to disregard a potentially problematic stain.

Please read our article on stains for more details, and feel free to get back to us with any further questions.