Why is a woman considered a niddah by declaring it? Does this apply when it is clear she is being sarcastic/does not mean it (such as saying it while pregnant out of frustration and there is no dam)? Can a delayed retraction in such a case remove the niddah status?
Thank you for your question.
The reason a woman is considered a nidda by declaring that she is a nidda is because she is believed. Therefore, the Aruch Hashulchan (Y:D 185:10) writes that if it is obvious from the way she said it that she didn’t really mean it, for example she said it jokingly, or as in your case she said it out of frustration, then she’s not considered a nidda. However, if when she said that she is a nidda it was not obvious that she did not mean it, she would be considered a nidda unless she gave a plausible explanation that explains why she said it even though she was really not a nidda.
All the best
Shulchan Aruch Y:D 185:3