With regard to menses, is it the period pain or the bleeding that counts as the signal to stop praying and fasting?

Praise be to Allah.

Firstly: 

What counts in order to determine whether a woman is menstruating or not is the appearance of blood. If bleeding occurs and the characteristics of the blood match those of menstrual bleeding, namely that it is dark, thick and has an odour, then in this case the woman is deemed to be menstruating. 

But if the woman feels some of the precursors of menstruation, such as pain, dizziness or feeling sick, but no bleeding occurs with that, then she is not menstruating and in that case she has to continue praying and fasting.

Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allah have mercy on him) was asked: I am a young woman in my 20s. Sometimes period pains come to me during Ramadan before Zuhr prayer, and it gets so bad that I cannot stand up to pray, and even if I sit down I cannot pray, but the period does not begin until five minutes before the adhaan of Maghrib. Please note that I continue fasting all day long. Is it permissible for me to make up that day or am I regarded as having fasted it? 

He (may Allah have mercy on him) replied:

If the period begins and blood comes out before the sun sets, then you should regard yourself as not having fasted, and you have to make up that day. With regard to the pains that occur before that, they do not invalidate the fast. The pain that signals the approach of bleeding does not invalidate the fast. If that pain continues but nothing comes out until the sun has set, then the fast is valid and you do not have to make up that day. But if that blood comes out before sunset, even if it is only five minutes before, then that day’s fast is invalidated and you have to make it up. This is the shar‘i ruling as far as we know.

End quote from Fataawa Noor ‘ala ad-Darb

http://www.binbaz.org.sa/mat/16927 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was asked: There is a woman whose period began shortly after sunset; is her fast valid? 

He (may Allah have mercy on him) replied:

This woman’s fast is valid, even if she felt the symptoms of menses before sunset, such as pain and cramping, but she did not see anything come out until after sunset. So her fast is valid, because what invalidates the fast is the appearance of menstrual blood before sunset, not just feeling it – i.e., the blood must actually come out. And Allah knows best.

End quote from Majmoo‘ Fataawa Ibn ‘Uthaymeen (19/270) 

Secondly: 

With regard to a woman inserting a piece of cotton or cloth into the place from which the blood comes out, if she does that before the menstrual bleeding appears – as is the case in the question – then that is not prescribed, because there is no report to that effect from the women of the Sahaabah (may Allah be pleased with them), and because it is a kind of obstinacy, which is blameworthy. 

But if that is done to check whether the period has ended, and the aim is to ascertain whether tuhr (colourless discharge signalling the end of menses) has occurred or whether the bleeding has stopped, then there is nothing wrong with this. The women used to send to ‘Aa’ishah (may Allah be pleased with her) a vessel containing pieces of cloth on which there was a yellowish discharge, and she would say: Do not be hasty until you see the white discharge.

Narrated by al-Bukhaari in Chapter on the Beginning and End of Menses 

Ibn Hajar (may Allah have mercy on him) said: With regard to the words “Chapter on the Beginning and End of Menses”, the scholars are agreed that the beginning of menses is known from the onset of bleeding at a time when it is possible for menses to occur. But they differed concerning the end of it. It was said that it is known when the bleeding dries up, which is when a piece of cloth inserted into the vagina comes out dry. And it was said that it is known from the white discharge. This is the view towards which the author is inclined, as we shall clarify below.

End quote from Fath al-Baari (1/420) 

And Allah knows best.