Behind the toilet is probably nice and dark and safe feeling to her. I can remember Wildfire building a nest in a cardboard play box in my closet while out for play time last year.
I would leave some extra litter boxes stuffed with hay for her to use. And she would pack the hay in her mouth and build a nest where she felt safe.
You can provide a box for her to build the nest in, if she builds it else where keep putting the nest in the box. They normally get the hint after a bit.
I had to remove the towell/blanket I had in Wildfire's cage because she kept dragging it in her litter box and building a nest with it. You may want to restrict her access to your clothes to prevent her from ruining them.
Provide her with lots of straw/hay to build a nest and a couple of boxes to build them in. Having her build it in a box means that you can easily pull the whole nest out to check on the babies and make sure none have escaped and are getting cold.
--Dawn