Am I sinful if I unknowingly err in distinguishing between menstruation (hayd) and irregular bleeding (istihadah)?

The default assumption for blood seen by a woman is that it is menstrual blood, as long as it occurs within a period suitable for menstruation. It is not considered Istihada (non-menstrual bleeding) unless it is seen during a time when it is not possible for it to be menstruation. A woman must consult scholars if she encounters a special situation whose ruling she does not understand, for it is not permissible for her to act based on her own opinion and desire.