Gender is a static thing for most, but not to those who are gender fluid. For them, the gender can change more frequently. Can change by the activity being done (so more consciously), or by the day (independently).
Typically, they prefer the they/them/their pronouns to be used, to prevent people from triggering them by assuming their gender incorrectly. Usually, they'll choose to be called by a gender-neutral name as well, but as in many cases, this varies from one person to another. Please go by their preferences in these matters.
Some may consider it rude to ask about their personal medical history concerning gender.