SERIOUSLY. Look at my bunny Sophie and your bunny Cassy. Could be sisters.
Soph is mini rex/rex too. Spaying or neutering is the best thing that you can do for your bunny. It ensures good health and that she will be happy and healthy for years to come. Bunnies are very clean naturally. Normally, they'll pick a corner of the cage/condo to make their bathroom and you can slip a litterbox right in there and they'll never notice. You also did wonderfully by putting hay in there since bunnies like to just hang out in their litterbox for fun.
Very improtant question- what kind of litter are you using? I highly recommend Yesterday's News snice its the only litter I have ever used.
Also, I always like to pass on this tip to new bunny parents that I discovered for myself. RABBITS FOR DUMMIES. It's my bible when it comes to everything bunny.