I'm not sure I've heard it either, but I don't think it too strange. When refering to a sovereign ruler proper I would think it wrong (since it should probably be 'Queen'), but if it refers simply to someone who is the sole leader of a specific group of people then I don't see why not. That said, Hatshepsut, a ruler in ancient Egypt is sometimes referred to as a 'female king' or 'woman king'.