What is the ruling on blood with varying characteristics that lasts for five to six days, and comes after a valid period of purity, for a woman suffering from severe polycystic ovaries, taking into account that at the beginning of the cycle it was light red blood, then it became black blood with an odor, and in the last three menstrual cycles it became dark red blood with black clots, and it has no odor, and it does not flow profusely, but sometimes stains clothes; and is it considered menstrual blood or irregular bleeding (istihada) according to the four madhhabs?