We just went through this at our house, and after a LOT of what turned out to be completely unnecessary procedures, a 14 day dosing of Clavomox cleared up the still undiagnosed ailment, which included vomitting and being off her food. I’m sure the vet has done a U/A on your kitty, tho that wasn’t very helpful diagnosing our kitty either. I get the need to rule out the super-serious stuff, but if your pet has been in generally good health and then just starts vomiting and loses the appetite, start ruling out the most obvious, least invasive, least expensive things first. We allowed blood work and an xray, then were pretty much badgered and guilted into an ultrasound - what ended up being a problem solved by a 2 wk course of Clavomox ended up costing close to 3 grand. And that’s because our vet went to DefCon 5 FIRST instead of starting out small and working up. Hopefully with the IV fluids, they have kitty on some general antibiotics to deal with unknown possible infections. Good luck! You are kitty’s protector - don’t let the vet take kitty OR you to the cleaners with unnecessary treaments.
Sometimes, kitties get sick. And a reason is never sussed out, but a round of meds can sometimes be just the ticket. Hope the resolution for your kitty is the easier and less expensive one!