Breastfeeding causes hormonal changes which generally suppress ovulation, leading either to amenorrhea (lack of menstruation), or to irregular bleeding. Amenorrhea while breastfeeding is even discussed in the Talmud.
The point at which regular ovulation, menstruation, and fertility resume varies from woman to woman. Therefore, it is quite common for women who are breastfeeding to have the kind of on and off staining you are describing. Whether or not staining makes you niddah depends on many factors. Please see our article on Stains for more details.
Although a pregnancy cannot be entirely ruled out, it is much more likely that your amenorrhea is related to your breastfeeding (as above). Research demonstrates that, for up to six months postpartum, women who breastfeed exclusively or almost exclusively and who have not resumed menses have less than a 1% chance of becoming pregnant.