Rabbit urine smells

a while ago had to move my outside rabbit indoors. its only recently that her litter box started smelling REALLY bad. So bad that i have to change it Every time she uses her box. She has fresh water, lots and lots of hay, hides and beds so i know its not a depression issue. I give her things to do when im gone and have her roaming when im in My room.

I want to say its a musky sort of smell so im thinking it could be a territorial/Mating season thing but i want to be totally sure.

I also have a cat that shares a room with her and sometimes she will go into the pen to mess around with the toys and my bunny hates it So recently my cat is only in my room when im in there and my bunny is put away at night so that my cat wont mess with her while im sleeping. I think i know the problem here but i just want to totally be sure.

any tips to make the smell less strong in the meantime while i save up money to get her fixed?