A nursing lady had a baby around a week ago and will be within 30 days of the birth on the 9th of Av. Does she have still have to fast? She has a difficult time fasting during nursing and in the past feels dehydrated rather quickly. I know that the Rema brings down that Ashkanazi women have adopted the Chumra of fasting after 7 days of birth, but does this apply today (even the Rama says one that is lenient has no loss) now that the aruch hashulchan and other poskim have brought down that we are weaker in our generation? If the Rav is lenient, should she drink or eat the minumum amount necessary and space the times like on Yom Kippur or does that not apply to Rabbinical fasts?
In principle, as you write, the Rema (554:6) writes that after seven days a yoledes tries to fast, and only breaks her fast if she feels tza’ar.
However, the Aruch Ha-Shulchan (554:7) writes that in our days people are weaker and within thirty days a yoledes should surely not fast. The Rema himself writes that one who is lenient does not lose out. This is also the common custom.
Although the Mishnah Berurah (554, Biur Halachah) mentions the concept of shiurim for Tisha Be’Av, in principle there is no obligation to minimize eating and drinking, and she can eat and drink as usual (certainly if she feels weakness).
Best wishes as always.