If a womens menstruation cycle had begun whilst fasting, How should she conduct her self and what if she stops menstruating during her fast?
In the name of Allāh, the Most Gracious, the Most Merciful.
Assalamualaikum Warahmatullahi Wabarakatuhu
If her menses commenced whilst fasting, She is permitted to eat (not in public). If her menses stopped in the day, she should abstain from eating, drinking, etc. in both instances she should make up for the Qada fast after the month of Ramadhan.
And Allah Ta‘ala knows best
Mufti Luqman Hansrot
 (Fataawa Aalamgiri, Rashidiyya, Vol 1, Pg 214)
(Ahsanul Fataawa, , Vol 4, Pgs 438, 457)
(Fataawa Mahmoodiya, Vol 10, Pg 169)