Having periods on your already infected vagina is one of the worse things to happen. While it might be genuinely uncomfortable to experience both simultaneously, treating your yeast infection during the periods is necessary.  Estrogen levels are low during the periods, and blood might help flush out some of that yeast infection.  Don't douch or do anything to aggravate the situation further but ask your doctor's help to treat yeast infection during periods. Sometimes bacterial vaginosis can also be confused with a vaginal yeast infection.