Mazal tov on your new baby!
Bleeding as a side effect of hormonal contraceptives does make you niddah. However, light staining does not make you niddah if it meets certain criteria. Please see the article on stains (ketamim) for a detailed explanation of these criteria.
The effects of progesterone vary from woman to woman. Some women (especially those who are breastfeeding) have no bleeding, some continue to have cyclic bleeding, and others have irregular spotting or bleeding. The first two possibilities generally do not present an unusual strain for observant women – but the last certainly can. There is no way to predict in advance how an individual woman will react. When starting the medication, you should be prepared to give it two months before deciding to discontinue, since many women who spot at the beginning notice a reduction in spotting after about two months of use.
Taking the pills at the same time every day can minimize irregular bleeding. Wearing colored underwear can help you avoid becoming niddah unneccessarily. But if you have a flow or experience hargashah, you become niddah even though this is from the pill and not a period.
Halachically, hormonal contraceptives are considered by many poskim to be the easiest to permit when contraception is allowed. For breastfeeding women, a progesterone only pill, such as Femulen, is generally preferred as it is less likely to cause reduction in milk supply. Most medical literature indicates that it does not affect the supply, but their have been exceptions. Nursing frequently is the best way to assure that your supply remains intact.
Many breastfeeding women do well on Femulen, so it may be worth trying. We suggest you begin the pills only after you have immersed in the mikveh, to avoid spotting during your shivah neki'im. The pills do not take effect immediately; however, you are unlikely to conceive at this stage and spermicide alone should provide sufficient protection. If after two months with Femulen, you are still having serious spotting problems, or if you do notice decreased supply as reflected in decreased weight gain in your baby (weigh her after about 2 weeks of use), then get back to us for further advice.
Another contraceptive method permitted by some poskim is the diaphragm. Rav Yehuda Henkin, the posek for this website, generally considers the diaphragm halachically preferable to contraceptive methods that are likely to lead to prolonged staining, such as the progesterone only pill. The diaphragm needs to be fitted by a qualified practitioner. It does not cause staining or affect milk supply.