Besha'ah tovah.

According to Torah law, a woman became a zavah ketanah if she bled for a day or two during her zavah days, the days outside of her usual menstruation, and the process for becoming tehorah was as you described. Nowadays, based on Rabbinic law, every woman who sees even a minute amount of blood treats herself as a zavah gedolah and counts seven clean days (after a five day minimum) before immersing in the mikveh.

The usual considerations that render a woman a niddah apply even while you are pregnant. Therefore, the circumstances under which you experienced the spotting will affect what your status is now. If you had no hargashah and no flow, and the only place you noticed the discharge was on colored underwear, you are not niddah (see article on stains for explanation). You should, however, consult with your physician if spotting continues.

If at any point during pregnancy you do become niddah, the steps for becoming tehorah are the same as when you are not pregnant.