Praise be to Allah.
Disruption of the menstrual cycle because of contraceptive pills
If the menstrual cycle is disrupted because of taking the pills mentioned, and you no longer have a regular period that comes at the time of your usual menses, with no regular time for its beginning and end, then in this case you must base your actions on distinguishing between the types of bleeding. What this means is that you should look at the blood that is emitted. Menstrual blood is black or dark, with an unpleasant odour, and is usually accompanied by pain (cramps); it is also thicker than the blood of istihaadah (irregular bleedings). Menstrual blood does not clot after it comes out, in contrast to the blood of istihaadah.
Based on that, then whatever meets the description of menstrual blood is menses, and at that time you should stop praying and fasting, and you should do ghusl when it stops, even if it appears intermittently throughout the month, because of the disruption caused by the contraceptive pills.
If the blood does not resemble menstrual blood from the outset, then it is istihaadah.
Among the reports which speak about distinguishing between types of blood is that which was narrated by an-Nasaa’i (215) and Abu Dawood (304) from Faatimah bint Abi Hubaysh, according to which she suffered from istihaadah. The Messenger of Allah (blessings and peace of Allah be upon him) said to her: “If it is menstrual blood, which is dark and recognizable, then refrain from praying, and if it is the other type, then do wudoo’, for it is only bleeding from a vein.” Classed as saheeh by al-Albaani in Saheeh an-Nasaa’i.
This applies so long as the total number of days on which bleeding occurs intermittently does not exceed the maximum duration of menses, which is fifteen days. If it lasts longer than that, then you should refrain from praying and fasting for a total of fifteen days, even if the days on which the blood matches the description of menstrual blood are not consecutive. Then whatever occurs beyond that timeframe comes under the ruling on istihaadah.
Please see the fatwa of Shaykh ‘Abd al-Kareem al-Khudayr regarding a similar matter in the answer to question no. 25784 .
Changing pads for every prayer in the case of istihaadah
The woman who is suffering istihaadah should wear something (a pad or sanitary napkin) that will prevent the blood from spreading, and she should do wudoo’ for every prayer; she does not have to change the pad unless she has not placed it properly and securely.
It says in ar-Rawd al-Murbi‘ (p. 57): The woman who is suffering istihaadah and others like her, such as one who is suffering from urinary incontinence, constant emission of madhiy or constant flatulence, or who has a wound that is constantly bleeding or has a continuous nosebleed, should wash the private part in order to remove whatever impurity is on it, then wrap it with something that will prevent spread of the impurity as much as possible. If it is not possible to wrap it, as in the case of haemorrhoids, he should pray as he is, and he does not have to repeat these two actions for every prayer, if he does his best. She should do wudoo’ when the time for each prayer begins, if anything has been emitted, and she may pray [with that wudoo’] so long as the time for the prayer has not ended, both obligatory and supererogatory prayers. If nothing has been omitted, then she does not have to do wudoo’. End quote.
And Allah knows best.