I asked on another board for you- and here were some of the answers:
has he been given anything for the fever to bring it down?? Blood tests indicate anything, like high white count? Are the kidneys tender?? Are fluids being given..and is output ok? What does the urinalysis say?
Also, did they do an ultrasound of the bladder...not everythin will show on xray...
Just some ideas, poor baby I hope they are at least treating the fever.
also another answer:
Could it be that she has an infection or if she passed the stone(your post did not indicate) that it scratched the bladder and caused an infection?
What does the vet think? Is she on antibiotics, I think she should be.
The board is staffed with vet techs that are very knowledgeable. I hope they have helped you figure this out.