Sheikh Sa`d al-Shuwayrikh
If a woman completes her menstrual period, then it is impossible for further blood to be menstrual blood. It seems that the blood in the case you you are describing is a continuous blood known as istihâdah and not menstrual blood.
When the woman sees the usual signs of purity, that means the menstrual blood has ceased. Any further blood should be considered istihâdah. The most important thing for the woman to do is to be sure that she has already seen the telltale signs of purity. If she does not see the usual signs of purity, further blood will still be considered menstrual blood.
The woman who is on her menstrual period is not allowed to pray or fast until she enters a pure state. Then she will be requested to make up whatever fasts that she has missed but not her prayers.
She will neither be requested to make up her missed prayers during the menstrual period nor the prayers that she made while menstruating under a false assumption that her menstrual cycle was over.
Source: Islam Today