She thinks it most likely that her period is coming; is it permissible for her to delay the prayer and not do it as soon as the time for it begins?
Question

                I am a woman; every month at a certain time, before my period comes, I feel pain in my back and my stomach, and I know that this is the sign of my period, and I expect that it will come at any moment, so I try not to delay any prayer, for fear that my period may come after the time for the prayer begins, and I will not be able to offer the prayer after it becomes obligatory for me. But nevertheless, my resolve weakens sometimes, especially with regard to ‘Isha’ prayer, because I usually delay it for two or three hours, and do not offer the prayer straight away. It has happened to me more than once that I delayed ‘Isha’ and did not pray straight away, and when I got up to pray it after two or three hours, I found that my period had begun. I know that on these days it is possible that my period could come at any moment. Am I sinning in this case? Am I regarded as deliberately not having prayed? I am not saying that I am not to blame, because I delayed the prayer, but I was weak and too lazy to offer the prayer straight away.
Praise be to Allah.There is a window of time within which the prayer may be offered, with a beginning and an end, so there is no blame on a Muslim if he prays at the beginning of that time or at the end. Jibreel (peace be upon him) taught the Prophet (blessings and peace of Allah be upon him) the times of the prayers, the beginning and end of each, then he said to him: “Whatever is between these two is the time [for prayer].” Narrated by Ahmad and an-Nasaa’i; classed as saheeh by al-Albaani in Irwaa’ al-Ghaleel (249).
But if a Muslim thinks it most likely that there will be some impediment that will prevent him from praying at the end of the time, then he must hasten to offer the prayer before that impediment arises.
Al-Mirdaawi (may Allah have mercy on him) said:
What is meant by his saying “it is not permissible to delay the prayer beyond its time” is that it is permissible to delay it within its timeframe, and this is correct, because undoubtedly the timings of the five daily prayers allow for flexibility, but our companions restricted that flexibility to cases where a person does not think that there will be some impediment that will prevent him from praying, such as death, being killed or menses.
End quote from al-Insaaf (3/26).
Al-Bahooti (may Allah have mercy on him) said:
He (or she) may delay it (the prayer) from the beginning of the time when it becomes obligatory, on condition that he (or she) has firmly resolved to do the prayer during that time, so long as he (or she) does not think that there will be something to prevent him (or her) from offering the prayer, such as death, being killed or menses. In that case, he (or she) must hasten to offer the prayer before that happens.
End quote from Kashshaaf al-Qinaa‘ (1/266).
Najm ad-Deen at-Toofi (may Allah have mercy on him) said:
The words “if he delays it (the prayer) even though he thinks he may die…” mean: if he delays a prayer for which the time is flexible, and does not pray as soon at its time begins, even though he thinks that he may die before he does it, such as if he thinks that he may die after the meridian by as long as it takes to do four rak‘ahs, and he delays it in spite of that, and does not hasten to do it when the time for it begins, then he is sinning merely by delaying it, according to the consensus of the scholars of usool, because he has delayed doing an obligatory action on time when he is able to do it and he thinks that he may die at the beginning of that time and not be able to make it up later on. In this case, he becomes like one who has been entrusted with something and fails to remove it from a place where he thinks that fire will reach it and burn it. The reason why it (failing to pray straightaway) is a sin, is because he has failed to do an obligatory prayer for which the time is flexible even though he thinks that he may miss it (if he delays it).
End quote from Sharh Mukhtasar ar-Rawdah (1/324).
Shaykh Zarakiyya al-Ansaari (may Allah have mercy on him) said:
The one who delays an obligatory prayer for which the time is flexible by not doing it the beginning of its time, for example, when he thinks that he will miss it because of death or menses and the like – which is more general than saying when he thinks that he may die – is sinning, because he thinks that he may miss out on the obligatory action if he delays it.
End quote from Ghaayat al-Wusool (19).
Conclusion:
It is not permissible to delay an obligatory prayer for which the time is flexible and not do it straight away when the time for it begins, except when one thinks it most likely that no impediment will arise that will prevent him from doing it within the time prescribed for it.
If a person thinks that such an impediment will arise, such as a woman who thinks that her period will come, because she knows its signs, then it is not permissible to delay the prayer until the end of its time; rather it is obligatory to hasten to do it before the time when it is feared that the impediment that will prevent her from praying will arise.
It does not seem that what you did comes under the heading of deliberately not praying, because you did that as a result of your misunderstanding and you hoping that the impediment would not arise and that you would be able to pray.
But if you had not resolved to do it within the prescribed time, then this does come under the heading of deliberately not praying, and there is no difference of scholarly opinion concerning that.
You have to seek refuge with Allah from helplessness and laziness, and hasten to pray when the time for prayer begins, if you are worried about the signs of menses that you mentioned. You also have to make up the prayers that you missed by delaying them in this situation.
And Allah knows best.