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.
The scholars differed in opinion about the minimum period of purity between two menses; many of them are of the view that it is 15 days while others are of the opinion that it is 13 days.
This second opinion is considerable in regard to its evidence and it is easier for you, so it is permissible for you to act according to it.
Therefore, if you see bleeding after 13 days or after 14 days from the last menses, then you should consider it as menses provided its period is not less than one day and one night, which is the minimum period of menses, and it does not exceed 15 days and nights, which is the maximum period of menses. For more benefit, please refer to Fataawa 91444 and 153821.
Allaah Knows best.