Women can’t fast during their period out of Allah’s mercy toward them, because loss of blood weakens them. If a woman were to fast on her period, she would be weakened both by menstruation and by fasting, in which case fasting would be an unfair burden and may even be harmful.

If a woman in nifas sees that she has become pure before 40 days are over, then she should do ghusl and pray and fast, and her husband may have intercourse with her.

It is haram for a Muslim woman to continue fasting when she is menstruating. If she gets her period while fasting, her fast is spoiled, even if the blood comes a moment before sunset, and she has to make that up if it was an obligatory fast.

Any bleeding that resumes after the period is over is not menstrual blood. So you can pray, fast, do tawaf or any other acts of worship.