Praise be to Allah.Lord of the Worlds.
First of all, we pray to Allaah to grant you healing and good health.
With regard to your question, if you become pure (i.e., your period ends) at the time of fajr (the pre-dawn prayer), then it becomes obligatory to perform ghusl (complete ablution) in order to perform the prayer. If you are not able to do ghusl because of your illness and inability to do it, then you should do tayammum (“dry ablution”). This means that you should strike bare earth once with the palms of your hands, then wipe your face, and the back of each hand with the palm of the other, then you should pray. And Allaah knows best. May Allaah bless our Prophet Muhammad.