Must a woman make up the prayers for the six days during which she stopped praying, believing it to be menstruation, only for it to become clear that she is pregnant? And is expiation incumbent upon her?

This small amount of blood is Istihada (bleeding outside of menstruation). According to the preponderant opinion that a pregnant woman does not menstruate, the matter is clear. As for the other opinion, it is also Istihada blood because the minimum period for menstruation is one day and one night. Therefore, you must seek forgiveness from Allah and make up the prayers you missed due to your negligence in inquiring, for Allah's debt is more worthy of being paid. When the menstrual blood stops, you must perform ghusl (ritual bath) and pray immediately, even if you think it will return, as your waiting for 6 days is a grave mistake. Strive to learn the rulings of the religion.