Does Bleeding Following Menstruation Invalidate the Fast?
Question

What is the ruling on bleeding from the vagina during the fast, when the monthly period ended ten days previously and the woman was suffering an infection in the womb? This blood is not like ordinary menstrual blood, rather it is like a short, thin thread and it happened once for a day and then stopped.
Praise be to Allah.This bleeding is not menstruation , so it does not affect the fast, so do not stop praying because of it. But you have to do wudu for each prayer. 
Shaykh Ibn ‘Uthaymin was asked: if a small spot of blood comes out of a woman during the day in Ramadan, and this (spotting) continues throughout the month of Ramadan and she fasts, is her fast valid? 
He replied: 
“Yes, her fast is valid, and this spotting does not matter because it is from a vein. It was narrated that ‘Ali ibn Abi aalib (may Allah be pleased with him) said: “These spots that are like a nosebleed are not menstruation.” (Sittun Su-alan ‘an Ahkam al-Hayd)
The Standing Committee was asked: Sometimes my period lasts nine days and sometimes ten. When I become pure (i.e., my period ends) and I get up to do housework, it comes back, starting and stopping. If it comes back after the time stated in shari’ah that a period may last, is it permissible for me to fast, pray and do ‘umrah? 
They replied: 
“The length of the period in your case is the length of time your period usually lasts, which is ten or nine days. When the bleeding stops after nine or ten days, then do ghusl and fast and pray, and do tawaf for Hajj or `umrah, or a voluntary fast, and it is permissible for your husband to have intercourse with you. Any bleeding that resumes after the period is over because of activities that you do or any other reason is not menstrual blood, rather it is due to some sickness or problem, so it does not mean that you cannot pray, fast, do tawaf or any other acts of worship. Rather you should wash it away, like any other impurity, then do wudu for every prayer, and pray and do tawaf around the Ka`bah, and read Quran.” (Fatawa al-Lajnah al-Daimah, 5/426)
And Allah knows best.