Chana declared ‘I will seclude myself with a stranger in front of my husband’ which will result in my drinking the Sotah water. How can she allow herself to transgress the prohibition of Yichud?
The Gemara says if one needs to fast on a Shabbos, he should nevertheless atone (for his neglecting Oneg Shabbos) by fasting again… If fasting on Shabbos requires atonement, how can one transgress a prohibition with the intention to atone for it later?
Pnei Yehoshua asks this question and answers that she did not mean to say that she will seclude herself merely that if Hashem will not listen to her tefillos then it is not right that a woman which transgresses and does seclude herself without sinning will benefit by having children whereas she (Channa) by being righteous and not secluding herself should lose out and not be blessed with children.
One is allowed to fast on Shabbos if it regarding a dream-taanes chalom- as it gives one pleasure to counteract the bad dreams-however he still needs an atonement for not enjoying the shabbos with food the way it is supposed to be enjoyed