Croup is a condition that develops quickly in children and is usually caused by a viral infection in the upper airways, which is localised in the throat and surrounding tissues. It's characterised by a barking cough. It's fairly uncommon and affects more boys than girls.
However, depending on the type of catheter you had installed (a urethral or a suprapubic catheter), you may still experience some catheter pain, especially in the case of an indwelling catheter that is expected to stay in your bladder for awhile â usually from a few days all the way up to forever.
Kidney failure is painful--if your vet didn't believe you I'd take her to a different vet clinic. They can give fluids, teach you how to give fluids, and what to watch for. Your description of her at this point is of a cat who is in pain and needs help. I'd get her in today to a vet.