Doctor of the Church means a teacher whose teachings are universally applicable to everyone in the Catholic Church. It has nothing to do with authority. Many nuns were traditionally teachers of children, too. And each of the female doctors of the church, as far as I know, where nuns who submitted all their teachings to their superiors and confessors, who then examined and verified her writings for orthodoxy (1 Timothy 2:14) before recommending them for others or permitting them to be published (cf. Imprimi Potest).