It could be for a few reasons

Some anaimals just do it (rabbits do it too), but you say shes not toliet trainned so it could be trying to hide it.

Apprantly also if they feel they lack nuritents they can do it, espicaslly if its runny they can think that it will make them better by eating it.

I would say if you try and sort out the toilet training and give lots of praise (and treats if you choose) when she goes in the right place then that should help the problem, I know it did with mine!

Best of luck!