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

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.