If you begin bleeding shortly after the fast of Tisha B'Av ends, you may count the day of Tisha B'Av as day 1 of the five day minimum, since you were continuously forbidden to have relations from the start of the fast until you became niddah.

Similarly, if you have an onat perishah on the night following Tisha B'Av, and begin bleeding at any point during that onah, you still may count Tisha B'Av as day 1, since here too you were continuously forbidden to have relations.

However, if you do not have an onah that night and begin bleeding later that evening or the following day, since there was a period of time in which relations were permitted after the fast ended before you began bleeding, you may not count Tisha B'Av as day 1 of the five day minimum.

Please feel free to get back to us with any further questions.

B'Hatzlacha!