Fathwa, - Confusion about the days of purity/impurity due to disruption in menstrual cycle

Question:
I have been using a chemical form of contraception for the past 6
months. It has changed my menstrualcycle, but for the first couple
months, it was easy to track. However for the past 2 months, Ihave
been bleeding slightly most of the time. I have considered this
istihaadah. However, I am confused now as towhen I should calculate my
haidh. My haidh usually lasts for 10 days, and the purityperiod in
between varies from 16 to 25 days. How do I calculate this?
Answer:
Assalamu alaykum
If one sees constant spotting less than fifteen days apart then it is
treated as if one has constant blood flow. In cases of constant
bleeding one has to rely on the length of one�s previous menstruation
and one�s previous purity period. One need not worry about one�s
�average� purity period, just the length of one�s most recent one. One
willthen assume that the length of one�s last menstrual cycle is one�s
menstruation and the length of one�s last purity period is one�s
purity and any bleeding seen in that period is to be considered
abnormal uterine bleeding (istihada). One will commence this
calculation of one�s menstruation from the end of one�s last normal
purity. (Radd al-Muhtar)
And Allah knows best.