Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
A woman is not permitted to make Istikharah while she is in menstruation or while recovering after child birth; she should limit herself to (Dua) supplications instead of Istikharah according to the Hanafi, Maliki, Shafii schools of thought. This suffices her Insha Allah.
Allah knows best.