We of course don´t want to speak here about typical average woman. Average woman is not warrior, thats undisputable.
But I can imagine some charismatic girls with bit more proactive and/or violent nature, living during some conflict (so basically anytime in medieval history) who lost her husband/father/whole family, and just doesn´t have anything else to live for. If she can learn something, she could in theory join the army, or MUCH more likely, become mercenary, vigilante, bandit…
Everybody here talks only about some official warriors, but becoming bandit(and later very well known bandit) is much more likely to happen after such events in this period. And there is quite a good reason to think that such bandits would have some physical skill and training with weapons, atleast to just survive, thus we can refer about such girl as warrior or fighter.
Although, I really think that life expectancy of such women must be really really short, as no lord, or city major would want to be ashamed by some woman and her thugs, ergo they would probably find and kill her rather quickly…