Have you looked into why this might have occurred? Did she get in trouble for something toilet related? Maybe there has been a big change in her life? (Family member died, change of daycare, house move etc). Might help trying to work out what might have occurred her to regress.

Sounds similar to my miss 4.5. She was doing toilets on the toilet great. Now she’s back to pooing her knickers. I don’t get it.

I wonder if you can find out what the trigger was for this then you can try and address the issues.
Maybe talk to her and ask gently why she doesn’t like her undies anymore.

Every child is different. I found with my kids when trying to encourage them I would offer rewards or take them shopping to buy some new undies that they get to choose, maybe some with their favourite characters on them.

Just be patient and don’t nag. It all takes time.

Regeression is a big word. Toilet training can simply go in stages.
In general I think it’s good to be patient, empathic and gentle when it come to toilet training.
She will get there at her time.
Factors like anxiety or stress, developing of fine and gross motor, hormonal balance, cognitive development and physical health can play a role.
For example do you know constipation can cause bedwetting ?

I had the same situation, in the end i just threw the nappies out, after a few wet nights it was over, I would say my fear of how hard it would be was definitely an issue my daughter barely showed any resistance once i had finally made my mind up , ,Cosi make great bed protector sheets for the interim

Sorry, but I never had this happen. Perhaps finding out the real underlying cause of why she feels insecure again may help.