I don't know for sure if this holds true for cats, but when a person has bright yellow urine it could mean that they are dehydrated. I would make sure kitty has plenty of fluids. Water especially but my vet told me that when they are dehydrated even chix or beef broth works. If you are unsure about the amount of fluids kitty is taking in I would check with the vet. He can always do an IV fluids if needed. Good Luck.