Spotting that appears on colored underwear does not usually make a woman niddah, as long as it does not turn into a blood flow.

You are correct that a hargashah would change the halacha, but physical sensations, such as cramping, that are often unaccompanied by any bleeding are not considered hargashot. Based on what you write, you are not niddah.

You may find it useful to review our site’s section on stains for further guidance on dealing with spotting. It may also prove helpful to consult your physician about your irregular bleeding. You should explain to him or her that, even if it is not a medical problem, it creates difficulties for you because of your religious observance. The section of our website designed for medical professionals, Jewish Women’s Health, includes relevant articles that can be shared with your doctor.

Our best wishes for continued good health!