Is a woman permitted to break her fast during the daytime in Ramadan if she inserts a cotton swab to confirm ritual purity and finds on it very faint traces of blood, after she had already eaten and drunk, assuming she was pure?

A woman's purity is determined by one of two signs: dryness or the white discharge. If she doubts her purity, the default is that she is still menstruating. If she finds blood when inserting a piece of cotton into the visible part of her private area, she is still menstruating, and you must break your fast; it is not permissible for you to fast. If she sees purity during the day, she must make up that day, and she is not obliged to refrain from eating for the rest of the day, although refraining is more cautious.