As Doris Day said, you can't make her poop in the potty. The more you try, the more obstinate the child becomes. I'd back off. I'd put her in a diaper and calmly tell her she'll have to wear a diaper so her clothes stay clean until she's able to poop in the potty. Be matter of fact and non-judgemental. Tell her she has a choice. Be sure to give her choices whenever possible in the rest of her life. Try giving her a hug and warmly tell her you know she can figure this out. You're there to help her. Increase giving her positive attention. Decrease negative attention, especially about potty issues.
I would be concerned about why this has happened. Has anything in her life changed? What is her mood like? What is the texture of her feces? Has that changed? We usually poop close to the same time everyday. Has her frequency and time of day changed?
You could watch to see her pattern and take her to the potty on a flexible schedule. You could try using the same way you trained her. Spend time having fun with her. Make using the potty fun. Perhaps it's time for her to use the big potty. Play up how using the big potty is for big girls.
I suggest that if you can't think of why she's changed and manage that and/or if anything else about her poop has changed you take her to the pediatrician. There could be something physically wrong.
An added thought. How long had she been using the potty? Sometimes a kid is not consistent in the way they poop. Is it possible she had an accident (s) and you were upset so now it is a power struggle.