Insha' Allah, you can answer this question about fasting during Ramadan. I was not fasting Ramadan today because I had my menses. Today, around 12 noon I discovered that the menses were finished. I had already eaten breakfast. I know that I should make Ghusl and pray, but should I fast the remainder of the day or start the next day?
All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger. We ask Allah to exalt his mention as well as that of his family and all his companions.
The scholars have agreed that a woman in menstruation when she becomes pure from her menses during the day in Ramadan, she has to make up for this day; the same applies to women in their confinement period. However, they differed as regards whether she should refrain from eating the remainder of that day. There are two different opinions about this. The most correct one is that she does not have to fast the remainder of that day and she can eat and drink. This is the view point of Imaam Maalik, Shaafi'ee and Imaam Ahmad and others. It is reported that Ibn Mas'ood, may Allah be pleased with him said: "Whoever had eaten at the beginning of the day, he is allowed to eat the remainder of that day." Therefore, the sister asking the question does not have to fast the remainder of that day, but she has to make it up another day.
Allah Knows best.