[Some Rulings on Menstruation]

Answer (and Allah grants success): If blood exits from a woman because of taking a medication, or due to an illness, and the like, then it is not menstruation unless it comes at the time of her habit; in that case it is menstruation. Whatever comes before her usual time or after it is not menstruation, and she must pray and fast.
The proof is the well-known report that the Prophet (May Allah bless him and his family and grant them peace) instructed the woman who complained to him of continuous bleeding to leave off prayer and fasting during the time in which she used to have her menses; then, when that time ended, to perform ghusl, pray, and fast, and so on. He (May Allah bless him and his family and grant them peace) also said to her: “That is but a kick from Satan, so do not abandon prayer and fasting.”
Accordingly, this sound report shows that when blood exits from a woman for a reason or due to an ailment, it is not menstruation; rather, she must pray and fast until the time arrives in which she used to menstruate—then, in that time, she leaves prayer and fasting, and her husband may approach her.
If the menses does not come at its usual time because she used a contraceptive treatment, and then comes ten days after her usual time and continues for seven, nine, or ten days, then—according to the school—that is not menstruation.
And if the number of her days increases because of using the treatment, the excess is not menstruation.
Likewise, brownish or yellowish discharge after the time of her habit—when caused by the treatment—is not menstruation.
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1