There is no Kaffarah but she has to repent and ask Allah for forgiveness. This is providing that she was certain and sure that she had her period. But if she was in doubt and had no room to confirm this, she didn’t do anything wrong.

If a girl experiences that her periods have started while she is at Haram or any masjid & later it is confirmed. What kaffara she has to make as she was in a state of major impurity & was sitting in the Haram?

Menstruation and post-natal bleeding |