Very light period on Pill

Even if your period remains very light, if you have a flow from your body, or if your bleeding is accompanied by a hargashah, or if you find blood on a bedikah cloth, that would render you a niddah.  A stain on a white garment (where you did not experience hargashah) could make you niddah if it's the size of a dime or bigger. If the color is doubtful, it should be evaluated by a rabbi.

If you find a stain on your body in an area where menstrual blood can reach (i.e., much of the lower part of the body or the hand)s, it could make you niddah, depending on its size and color.  The color of a stain on the body can be evaluated if you transfer the stain onto a cloth (by pressing the cloth to the body).  Whereas distinct stains on a garment are each treated independently with respect to size (i.e., does any one stain exceed the size limit?), distinct stains on the body are evaluated together, by their total area (i.e., does the aggregate size of these stains exceed the limit?).  For a review of the laws of stains, see our article "Stains (Ketamim)". 

If your period does not meet any of the above conditions, then you would not become niddah.  There is no obligation to be niddah – only to observe the relevant halachot when you are. Women who go from pregnancy to nursing to pregnancy can end up being niddah only once every year or so.

We do recommend that you abstain from intercourse until about 24 hours after the staining subsides so your status can be clarified. This is a precaution, not a halachic requirement.

The mid-cycle spotting that can occur with hormonal contraception can sometimes also render a woman niddah. It is subject to the same rules mentioned above.