In the answer to question no. 68818 we explained when bleeding is menses (hayd) and when it is isthaadah. When it is menses then it is subject to the rulings on menses, and when it is istihaadah it is subject to the rulings on istihaadah.
The main rulings on menses have been mentioned in the answer to question no 70438.
The rulings on istihaadah are like the rulings on purity (i.e., when one is not menstruating). There is no difference between a woman who is experiencing istihaadah and a woman who is not menstruating or bleeding, except the following:
1 – She has to do wudoo’ for each prayer, because the Prophet (peace and blessings of Allaah be upon him) said to Faatimah bint Abi Hubaysh: “Then do wudoo’ for every prayer.” Narrated by al-Bukhaari in the chapter on washing away blood. What that means is that she should not do wudoo’ for a prayer that is to be performed at a particular time until the time for it has begun. If it is a prayer for which there is no particular time, she may do wudoo’ for it when she wants to perform the prayer.
2 – When she wants to do wudoo’, she has to wash away the traces of blood, and put a piece of cotton over her private part to hold back the blood, because the Prophet (peace and blessings of Allaah be upon him) said to Hamnah: “I suggest you use a piece of cotton, for it will absorb the blood.” She said: It is more than that. He said: “Use a cloth.” She said: It is more than that. He said: “Then tie it tightly around yourself.” Then whatever comes out after that will not matter, because the Prophet (peace and blessings of Allaah be upon him) said to Faatimah bint Abi Hubaysh: “Do not pray during the days of your menses, then wash yourself and do wudoo’ for every prayer, then pray, even if the blood falls onto the mat.” Narrated by Ahmad and Ibn Maajah.
3 – Intercourse. The scholars differed as to whether it is permissible when one is not concerned about suffering hardship as a result of giving it up. But the correct view is that it is permissible in all cases, because many women, ten or more, experienced istihaadah at the time of the Prophet (peace and blessings of Allaah be upon him), and neither Allaah nor His Messenger forbade them to have intercourse. Rather the words of Allaah – “keep away from women during menses” [al-Baqarah 2:222] – indicate that it is not essential to keep away from them at other times. Also, it is permissible for her to pray, and intercourse is a less serious matter. Drawing an analogy between intercourse with a woman experiencing istihaadah and a woman who is menstruating is not correct, because they are not the same, even in the view of those who think that it is haraam, and analogies are not valid if the two things concerned are different. End quote.
Risaalah fi’l-Dima’ al-Tabee’iyyah li’l-Nisa’ by Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him).