If you were to doubt the onset of menstruation during the day in Ramadan before the Maghrib call to prayer, would you be obliged to invalidate your fast, and would what you did be valid?

If a fasting woman doubts the onset of menstruation, she must continue her fast until she is certain of menstruation. This is because the default state is purity until menstruation is definitively established. What she did is correct. It is stated in Mughni al-Muhtaj: "Because the default is the absence of menstruation."