Experience with Fever of Unknown Origin?

Hi, all.  I'm really worried about my 1 year-old kitty, Sirius Black.  Overall he's been a healthy kitty.  When I came home from work yesterday I noticed that one of my cats (Sirius has a brother, Oli) had vomited.  One of them had diarrhea as well.   Sirius began acting very lethargic and not like himself so I took him to the vet late last night.   Poor little guy had a fever of 105.  The vet gave him an injection of antibiotics and asked me to bring him back this morning.  He acted very unwell this morning....I could tell he was uncomfortable and he was being very vocal (he's a vocal cat anyway but this was different).

He's now at the vet while I sit at work unable to concentrate!.  Apparently his temp hasn't subsided but his x-rays and blood work are normal.  Vet is really concerned (as am I).  He's receiving IV fluids today and I'm assuming more antibiotics.  She told me we have entered the fever of unknown origin stage.  Oh I should mention that he was also tested for feline leukemia....he was negative when he was a kitten but the vet said they can still test positive later in life.  

Has anyone else had an experience like this??  I'm really freaking out....I'm worried it might be FIP.  

Any advice or info you can provide would be greatly appreciated.  I'm just so worried about my little guy.  

I'm so sorry for you and Sirius.  I hope the vet is able to figure out what's going on.

Thanks....I hope so too!

Yes, my cat has a fever of unknown origin last month. My cat eventually recovered with antibiotics. Have you done any urine testing or uninalysis? Could it be undetected UTI? The human urinalysis strips work with cats, so you can buy some or have the vet take a urine sample. Are his stools normal in color? And is he still eating and drinking on his own?