I think a compelling example might be the Lady Jane Grey who at least partially fits into the contours of your question.

Lady Jane Grey was declared Queen of England following the death of Edward VI in 1553. This was clearly a controversial decision amongst contemporaries however the King had chosen to pass the crown to her for number of intriguing reasons. First, as a devout Protestant he wished to pass on the crown to someone who shared in some sense his religious convictions and opposition to Rome; the Lady Jane had come of age in a deeply Protestant intellectual milieu and seemed a sound choice to continue his religious reforms. She was also a Tudor descendent of Henry VIII's sister Mary and thus fit into a valid line of succession outside of the principle heirs stipulated by the Third Succession Act which would have preferably seen Mary, the Roman Catholic daughter of Catherine of Aragon, named his successor. As he is said to have remarked on his deathbed, "Should the Lady Mary of the Lady Elizabeth succeed she might marry a stranger, and the laws and liberties of England be sacrificed, and religion changed." (Chapman, The Last Tudor King, 180) Such a threat to his reformed principles was difficult for Edward to bear, and he was likely convinced to name Lady Jane by his chief minister who was Jane's father-in-law.

Unfortunately for Jane her succession was immediately contested by members of the nobility who disapproved of this deviation from the succession act, a number of whom were Catholics who had maintained their faith in secret at the Protestant court.

Lady Jane, now Queen Jane, had limited support and faced with overwhelming opposition was quickly arrested, tried and then executed. The Catholic claimant Mary ascended to the throne with the support of the privy council and for a time changed the religious and political trajectory of England back towards the Catholic world and towards close ties with the Spanish monarchy.

You asked in your question for women who had a big impact, and obviously her reign at roughly nine days had a limited one. However I think she is nonetheless worth reflecting upon given her limited public visibility in contemporary society and general anonymity.

However, had she received the full support of the English nobility the range of possible counterfactuals are compelling, and could have changed the character of English Protestantism going forward and England's relationship with neighbouring powers on "both" sides of a blurred and conflicted confessional divide (to put it very simply indeed).

We should recall that her fate was not necessarily sealed at the time of Mary's uprising. It was only after a Protestant rebellion broke out against Queen Mary that violently opposed the imposition of Spanish Catholic influence in the English court, which signalled her demise. She became a kind of symbol of the dangers of Protestant claimants to Catholic monarchs, and her execution became part of a wider pattern of persecution that claimed the lives of numerous Protestant clerics and lay reformers caught up in this new period of political and religious turmoil.

Anyways, I hope you found that somewhat interesting nonetheless and that others provide you with additional answers that more specifically address your question.

Eric Ives, Lady Jane Grey: A Tudor Mystery
Chapman, The Last Tudor King

When you say female monarchs, are you counting all Queens, or only Queen Regnants (ie Queens in their own right, those being Elizabeth 1+2, Mary 1+2, Anne, Victoria, and arguably Matilda)? Are you asking about Queen Consorts too (ie the woman who is called the Queen because she's the wife of the King, but doesn't have her own legal power to rule)?