What is the ruling on the fast of a woman who experienced light bleeding outside of her regular menstrual cycle, and the bleeding continued while she was praying? Is it permissible for her to complete the six days of Shawwal fast?

The default assumption for blood discharged from a woman, not due to childbirth, is that it is menstrual blood, unless proven otherwise.

If there is a period of 15 days or more between the onset of the blood and the end of the previous menstruation, then it is menstrual blood (hayd), which prohibits prayer and fasting. Fasting in this case is not counted.

However, if the blood appears before 15 days have passed since the end of the previous menstruation, then it is irregular vaginal bleeding (istihada), which does not prohibit prayer or fasting.