You may attribute the stain found on your underwear to a vaginal irritation since this is common when experiencing a painful yeast infection. Your clean days were not interrupted. While you should change your white underwear more frequently to prevent any staining from accumulating to a gris, any stains on your underwear (even if larger than a gris) may be attributed to the irritation as long as your situation does not improve.
However, while external stains may be attributed to trauma, it is more complicated to attribute stains found on bedikot. Therefore, we recommend omitting bedikot until day 7, and then very carefully and gently performing a bedikah on that day. It may be a shallower than usual bedikah if necessary, as long as your finger enters until the first knuckle.
Please feel free to get back to us with any further questions.
Refuah Shleimah!