Question: What does a woman do if her menstrual bleeding stops and then returns back after a few days?
Answer: She should take a ghusl (a bath) after her bleeding first stops, and pray, fast and have relations with her husband. Then if she starts menstruating again, she stops praying, fasting, etc. Then when she stops bleeding and becomes pure from it, she bathes and prays and fasts once more. That is because a woman’s monthly cycle (period) may increase in days, or decrease, come in one time-period of days, or be intermittent with gaps.
See Al-Jāmi’ fī Fiqhil-‘Allāmah Ibn Bāz (p. 97).