It is quite common to experience irregular staining during the peri-menopausal stage.
As long as you do not experience an actual flow, you may take precautions against becoming niddah from the staining by wearing colored underwear or disposable pantyliners, and waiting at least a few seconds (ideally 15 seconds) after urinating before wiping in order to disregard stains found on toilet paper.
Even if the stain is accompanied by cramping, that is usually not considered a hargashah, and it would not render you niddah.
Hargashah has a very specific definition in halacha. It is defined as one of three sensations – feeling the uterus opening, feeling one’s whole body shaking, or feeling liquid exit the uterus into the vagina. These sensations are quite rare, and unless a woman is certain she experienced one of these sensations, she can assume she did not experience a hargashah.
Sometimes, a woman may experience a specific symptom that consistently precedes her periods. This may be considered a veset haguf if she consistently experiences this symptom at a set time/interval preceding the onset of the flow, and she never experiences this symptom at any other time other than before her period. For example, if you always begin bleeding 12 hours after feeling cramping, and you never experience this type of cramping at any other time, you would be required to perform a bedikah 12 hours after feeling this symptom. If the symptom does not fall into the above parameters, it is not considered a veset haguf, and has no halachic significance.
Even if the stain is large, if it is not a flow, it is subject to the leniencies of stains. While it can be difficult to distinguish between heavy staining and a light flow, one way to distinguish is if you need to use a pad to contain the bleeding (as opposed to a light pantyliner). If that is the case you should assume you are experiencing a flow. Bleeding lighter than that is likely just heavy staining.
Based on your description of the stain you found, if it was found on colored underwear or on toilet paper (not immediately after urinating) it did not render you niddah. We do recommend abstaining for 24 hours after finding a stain so your status may be fully clarified.
Please feel free to get back to us with any further questions.
This response has been updated to reflect the rulings of our current Rabbinic Supervisor, Rav Kenneth Auman, regarding waiting before wiping.