All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
The waiting period for a woman who has menses is three menstruations even if the period of her menses is long, as Allaah determined the waiting period by the occurrence of three menses, as He Says (which means): {Divorced women shall remain in waiting [i.e. do not remarry] for three menstrual periods.}[Quran 2:228]. So it is not permissible to ignore this rule and count three months except when the woman stopped to have menses or a girl who is below the age of puberty. Allaah Says (which means): {And those of your women as have passed the age of monthly courses, for them the 'Iddah (prescribed period), if you have doubts (about their periods), is three months, and for those who have no courses [(i.e. they are still immature) their 'Iddah (prescribed period) is three months likewise,…]}[Quran 65:4]. So the waiting period for women who have menses is by having three menstruations and those who have no menses is three months; this is either for those who have passed the age of having menses or those who are still immature.
Finally, it should be noted that using pills to prevent pregnancy has many disadvantages, and causing irregularity to the monthly periods is one of these disadvantages, so a woman should not use these pills except in case of a necessity. As regards the blood that might come out after becoming pure from menses, then if it comes out after 15 days then this is menses, as the minimum period between two menses is 15 days according to the majority of jurists. But if this blood comes before the end of a period of 15 days after becoming pure [from menses], then this is not considered as a second menses, it could be that this relates to the last menses or blood outside the period of menses.
Allaah Knows best.