On the question of Thuhr following menses, let me first cite here one of my earlier answers:

“ You are free of menses only when your bleeding has completely stopped, and you see the white discharge, following your regular pattern. However, after you have seen the white discharge, if you experience pinkish or brownish discharges afterwards, you need not worry. Umm Atiyyah, the Prophet’s companion, said, ” During the Prophet’s time, we never used to pay attention to the pinkish or brownish discharges that followed the expiry of menses.”  (Reported by Imam Bukhari and others).

All the above, however, is conditional on the fact that your menses period did not exceed fifteen days. For bleeding after fifteen days cannot be reckoned as menses; rather it is reckoned as istihadhah (abnormal bleeding). In the case of istihadhah, you need to pray after having taken a full bath at the end of your period; then you wash your private and make wudhu before each prayer. Washing and wudhu should only be done immediately prior to each salah.”

As for any confusion in regards to the number of days, it should be settled by considering the nature of bleeding, as well as the number of days you used to experience in the past. Menstruation can never exceed more than fifteen days. If you have had a pattern of menstruating six or days, then that should be standard for you to calculate the period.  Any bleeding over and above the standard is ruled as being due to a chronic condition. You may also seek the advice of the gynecologist.

The scholars ruled on such matters based on empirical observation in their times. Today science can answer such questions far more accurately. Therefore, you should consult your gynecologist; she should be able to sort out the confusion for you.

Almighty Allah knows best.