What is this?

This morning I saw this in my female bunny's litter box. Looks like pressed paper. Does anyone know what is it?
Context: I have 2.5 year old female bunnie. She is free in my 20 square meter house and at night I put her in a big dog cage which is double the size of her litter box. She is not neutered and she doesn't have access to any type of paper during the night. This thing wasn't there when I went to sleep.

Pls help