Somehow she get scared while was at the right place to do it? Or injured in something near? Or the place was moved out? Sometimes this could happen and dogs (and cats too) became confusedn and afraid!

Could help too if you pick up the poop and put it at the right place where she is suposed to make it, an than you just show her where the poop is placed, gentle without anger voice but firm!!

This usually works with cats and dogs!

Stay well!