S.H.
Ditto Denise P.
Power struggle or not... is a child starts to "withhold" their poop, it can become a very difficult MEDICAL condition... leading to -constipation, and "encopresis." These things will need medical intervention.
When my daughter was potty training, she started to withhold her poop. We never forced her to go on a toilet nor pressured her for pottying.... but it just made her feel anxious. Then she didn't want to poop. A child, will OFTEN "master" pooping, later.
We had to see a Pediactric Gastroenterologist... HE said: do NOT pressure a child emotionally, just to poop. Because it causes all sorts of medical problems. And the more they withhold their poop, the harder it gets, then it is PAINFUL for it to come out... and because it 'hurts' the child will NOT want to poop at all. At all. Then they get bulging bowels and constipation gets worse, and Encopresis can occur. Which is an involuntary medical condition.
HE said, that for my daughter, her 'constipation' and withholding of poop, may take even 3 months... to correct. We also had to give her prescription of things to correct the consistency of her poop... and THEN, the child has to overcome the "emotion" based 'fear' of pooping because it hurts and because it is just a negative association.
Anway, don't pressure her. Don't turn it into a battle, just to poop. Let her go in a diaper. No biggie. She obviously is not 'ready' for any of that. Thus 'regression.' Regression in a child is a symptom of 'stress' or their inability to cope with something.
She is already 'withholding' her poop. This is not healthy and can lead to other medical problems, as I mentioned.
all the best,
Susan