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 minimum period that is between two menses is 15 days according to the view of the majority of the scholars so if this girl sees this blood 15 days or more after its period stopped – as we understood from the question – then this blood is considered a second menses if this blood lasted for one day and night or more, in which case, she is obliged to refrain from performing the prayer and fasting as is the case of any menstruating woman, and she should perform Ghusl when her blood stops. If the blood returns again, she should again stop from performing the prayer and fasting and so on until her blood stops completely as she does now. However, if she sees this blood after less than 15 days after her previous menses stopped, then there are some details about it as there is no need to mention it now.
For more benefit, please refer to Fatwa 100850.
Allaah Knows best.