I'm a nanny and a little girl I used to look after did exactly the same as your little girl. She was probably 3 or 4 months younger at the time but had been using the loo for everything and then started doing poos in her knickers. We found that sometimes she was unaware that it had happened - she was so busy playing and having fun that it didn't seem to bother her that she had poo in her knickers!! After handling with kid gloves, getting cross, ignoring and just changing her, and all failed to make a difference we went down the smartie reward route. She was definitely reward driven and an instant chocolate was a good way to go. We also bought new knickers with her favourite character on them. She could pick a pair to wear in the morning but if she had an accident in them then she had to choose an old pair to wear and we made a huge deal of 'peppa' or whoever it may be being very sad about being poo'd on! We also asked every half hour or so if she needed a poo and reminded her of a smartie and not upseting 'peppa'. After a week or so she seemed to really get the hang of it again and everything settled down. Hopefully your little one will get back on track soon. Good luck!!