Her menses ended but there is still some blood in the urine; should she fast?
Question

After my menses came – and actually I do not know how many days it lasts – on the day when the menses stopped, there were still some drops of blood in the urine. I did not fast on that day, on the assumption that it was menses. Do I have to make up that day, and is there any sin on me, or was it permissible for me to not fast?
Praise be to Allah.The end of menses can be determined by one of two signs:

Emission of the white discharge, which is well-known to women.
Complete dryness, so that if a piece of cotton or the like is inserted into that place, it comes out clean with no trace of blood or yellow on it.

Al-Baaji said in al-Muntaqa Sharh al-Muwatta’ (1/119): What is usual when the menses ends is two things:
[The first is] the white discharge. ‘Ali ibn Ziyaad narrated from Maalik that it resembles semen. Ibn al-Qaasim narrated from Maalik that it resembles urine.
The second is dryness, which means that when the woman inserts cotton or a piece of cloth into her vagina, it comes out dry, with no trace of blood on it. What is normal for women varies; some of them usually see the white discharge, and some of them usually see dryness.
So for a woman who usually sees one of these two things, and she sees it, then that is deemed to be her tuhr [becoming pure when menses ends]. End quote.
If she does not see the white discharge, or does not become completely dry – rather there is still some blood, even if it is little, or there is yellowish or brownish discharge – then she is still menstruating, and it is not permissible for her to fast. You have to make up that day, but there is no sin on you, because you were following the Islamic command of not fasting at the time of menses.
The basic principle is that the bleeding that occurs at the usual time of a woman’s menses is menstrual bleeding, so long as it does not go beyond fifteen days; in that case [if it lasts longer than fifteen days], then it is istihaadah [irregular, non-menstrual vaginal bleeding] according to the majority of scholars.
It should be noted that menstrual blood does not come out of the same place as urine; but perhaps what you meant is that you saw the blood, or you noticed it, when urinating.
And Allah knows best.