Missing a pill can sometimes lead to breakthrough bleeding. Since breakthrough bleeding is uterine, it can make a woman niddah.
Ashkenazi decisors are more stringent than sefardi halachic decisors on this issue, so if you are sefardi, you may disregard what you saw. If you are ashkenazi, then, since the stain was directly on the toilet paper and was red, you may be in niddah. (Please get back to us to clarify if you are not sure if the stain was on the paper.)
For a woman who follows ashkenazi rulings on niddah, the next question is the interval that elapsed between urinating and wiping. If the interval was at least a few seconds, then the stain may be disregarded. If you wiped immediately after urinating, you are niddah unless you have some physical irritation (such as hemorrhoids) to which the staining might be attributed.
For more information, please see our site’s article “Toilet Paper“. Please write back with any follow up questions.
This response was updated on 4 February, 2024.