If it hasn't been 24 months and there is considerable food from her for the baby or the milk may stop, it can be relieved, and if you feel weak, you don't need to fast at all.
Sources: This should be preceded by the principle of law that a breastfeeding woman is exempt from fasting, because she does not have to observe fasting and there was no regulation for fasting at all, only that they used to observe fasting (see Rama 3:19 1961 and Shnab Skav in the name of the arbitrators), but in the case of breastfeeding today, Some of the arbiters of our time instructed that since today the need of the nursing woman is very great as will be explained, therefore she can eat, according to the MS Mishnab SKA that if they are weak they should not get worse.
And the issue of the exemption that a breastfeeding woman has while fasting is not from the main point of the custom mentioned in the Rama, since the Rama took that they used to make it worse on themselves, but today some of the arbitrators eased this is because the condition of the breastfeeding woman today should be considered as weak or because of the small necessity that she is a little sick.
However, in the event that many months have passed since the birth and besides that, the main part of the child's eating is not from the mother and is not consumed by her at all, he strongly urges to make it easier, (and according to Solomon's Walks between the Egyptians 57 regarding the Halacha what he ordered after 9 months).
And it is true that if the woman feels weak and it has not yet been 24 months after giving birth, there is room for relief (cit. Khot Shani Shabbat 44 p. Res and Hagg Laws in Hagg 1512), and likewise if a significant part of the baby's nutrition comes from the mother (cit. second thread there), or ACP in the event that there is a fear that if the woman fasts her milk for the baby will stop, in such a case one has to weigh it (Shumoot Purim rulings, p. 11 in the name of the Garchak).
And it is true that with regard to the laws of Shabbat, there are those who have taken the view that all small needs are considered as the needs of a sick person (i.e., what we brought in the name of the Gra in one of the adjacent answers), and there are also those who have taken with regard to ablution in the Hohamim, as well as regarding the issue of ablution among the Egyptians according to the custom mentioned by the Rama (Takna SS hand), but for our purposes the boundaries are not clear in this and we recall that those who are weak will not comply with the lesser known boundaries to consider what is considered a minor necessity, but there is indeed logic that if there is one of the above conditions mentioned in the name of the second thread and the Garhak, it should be considered the minor necessity for the purpose of permitting a woman .
And it should be noted that the permission to breastfeed in our times in such a way that there is no obvious and known weakness is not simple and is not agreed upon (see the examples of places in the comments and addendums to Rish C. 19), and that several instructions were brought forth in the name of the Garshaza on the matter, according to which it seems that he ordered in any case according to the matter of The same case, and in fact it seems that since the main thing discussed about milking is a custom, and it should be mentioned that the law of these fasts in our time is also a law that Israel accepted for them and they do not have a Durbanan rule like other regulations such as Ma'o in Gm and Ramban and Tor, and they did not accept about milking and so on, therefore there is more There is room to ease this, and also in the second thread it is seen from his words that he eased the instructions in this regard.
Therefore, in NIDD, if it has not been 24 months and there is considerable food from her for the baby or the milk may stop, she can make it easier, and if she feels weak, she does not need to fast at all.
Regarding what is allowed to be eaten by those who eat while fasting, they shall not eat except for the necessity of the baby's sustenance (see Shua Takand Saha).
But you don't have to eat lessons like in the Bible (see the second thread there, p. Ranach).
And you will eat in Sinai (see Meta Ephraim 3. Tarev Sakhab).
In the framed article it should be noted that now in this period there is war in the USA and not peace, therefore there is a question about all of our public fasts in any case if they are a complete public fast, and for our purposes also regarding breastfeeding since the obligation is complete anyway it should be made less easy, but in fact it seems that the custom that they were not afraid of it, even during the first times when there were persecutions and exterminations, they did not state that they practiced on every 4th day of fasting as in the matter of the other laws of public fasting, except for Ba'al Nefesh mentioned in the NKJV Shu'a according to the arbitrators, regarding the matter of locking the sandal, And I heard that Gerash Devlatsky practiced a complete fast during the war, and if from the above-mentioned country or from a country accepting fasting he accepted a public fast, however, the established custom from generation to generation is not like that, and therefore I did not bring the body of the matter in this regard.
מק"ט התשובה הוא: 4342