What is the ruling regarding someone who woke up after the Fajr Adhan, did not have suhoor, and does not know whether she intended to fast for a missed day (qada') or not? And should she fast or not?

Fasting for making up missed fasts (qada') is not valid unless the intention (niyyah) is made during the night. A woman waking up for suhoor (pre-dawn meal) is considered an intention. The intention for fasting is easy to make. So, whenever a woman intends to fast, let her fast and not be bothered by whispers (waswas), for the doubt of one afflicted by waswas is not considered. "Doubt after an action has no effect, and similarly when doubts become numerous."