Can I Pray If I Am Spotting After My Period?
Question

I wanted to know how a woman should determine when to start praying after her menses. What should she do if she thinks she has finished her menses and starts praying and then later she sees more blood or a brown discharge?
Praise be to Allah.When does a period end?
When a woman menstruates, her period is over when the blood stops, regardless of whether the blood was a lot or a little. Many jurists stated that the shortest length of a woman’s period is a day and a night, and the longest is fifteen days.
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said there was no minimum or maximum for it ; when the bleeding is present with its distinct qualities, this is menstruation, whether it is a little or a lot. He said:
“Menstruation (al-hayd): Allah has attached numerous rulings to it in the Quran and Sunnah, but He did not state the shortest or longest length (of a menstrual period), or the length of the period of taharah between two menstrual periods, even though the need to know that exists.”
Then he said:
“Some of the scholars defined a maximum and minimum, but they differed concerning that, and some stated a maximum length without defining a minimum. The third view is most correct, which states that there is no minimum or maximum.” (Majmu’ al-Fatawa, 19/237)
What is istihadah?
There is a kind of bleeding which is called “al-istihadah ”, [irregular, non-menstrual vaginal bleeding] which is different from menstrual bleeding. It is governed by rulings which differ from those governing menstruation. 
Difference between istihadah and menstruation
This blood (istihadah) may be distinguished from the blood of menstruation by the following:

Colour: menstrual blood is black [dark red] whilst the blood of istihadah is red.
Consistency: menstrual blood is thick and heavy, whilst the blood of istihadah is thin.
Smell: menstrual blood has an offensive odour whilst the blood of istihadah does not, because it comes from an ordinary vein.

These are the characteristics of menstrual blood, so if this description fits the blood that is coming out, it is hayd (menstruation); ghusl is wajib (obligatory) and this blood is najis (impure), but ghusl is not required in the case of istihadah.
When a woman has her period, she is not allowed to pray, but when the bleeding is istihadah, that is not the case. But she has to be careful and has to do wudu for each prayer if the flow of blood continues to the next prayer. If blood comes out whilst she is praying, it does not matter.
How to know your period is over
A woman can know that her period has ended by two things:

The white discharge which comes from the womb to show that the period is over.


Complete dryness, if a woman does not have this white discharge. In this case she can find out if her period has ended by inserting a piece of white cotton or something similar into the place where the blood comes from; if it comes out clean, then her period is over and she has to do ghusl and pray. If the cloth comes out red, yellow or brown, she should not pray.

The women used to send small containers in which were these cloths with traces of yellow on them to ‘Aishah, and she would say, “Do not hasten until you see the white discharge.” (Narrated by al-Bukhari mu’allaqan. Kitab al-Hayd, Bab iqbal al-mahid wa idbarihi. Also by Malik, 130).
Does brown blood mean your period is over?
But if there is a yellow or brownish discharge when a woman does not have her period, this does not mean anything and she should not stop praying or do ghusl because of it, because it does not necessitate ghusl or cause janabah (impurity).
Umm ‘Atiyyah (may Allah be pleased with her) said: “We did not bother about any yellow or brownish discharge after a woman’s period was over .” (Narrated by Abu Dawud, 307. It was also narrated by al-Bukhari, 320, but he did not mention “after a woman’s period was over.”)
“We did not bother about” means we did not think it was menstruation, but it is a najis (impure) discharge which means that a woman has to wash it away and do wudu. But if the white discharge is immediately followed by a period, then it is part of the period.
Can you pray if you are spotting after a period?
If a woman believes that her period has ended, then she starts bleeding again, if the blood has the characteristics of menstrual blood as described above, then this is menstrual bleeding , otherwise it is istihadah.
If it is the former, she should not pray. If it is the latter, she should be careful and do wudu for each prayer, then pray.
Concerning the brownish discharge , if she sees this after her period has ended, then the ruling is that it is tahir (pure), but it requires her to do wudu only. But if she sees it during the time of her period, then it comes under the rulings of menstruation.
And Allah knows best.