[Alayambo]

A bit of back-story:

One of my cats (Kiri) recently had surgery to correct some pretty serious tooth decay issues, and ever since then my other cat (Killer) has been very aggressive, which I think may be due to non-recognition. Before the surgery Kiri had some pretty bad breath, and I'm thinking it's possible that Killer doesn't recognize her because the smell is no longer there?

At first Killer was full-on attacking Kiri; yowling, hissing, biting, scratching; and even though I always broke it up quite quickly, Kiri started feeling the need to fight back, to the point of growling/hissing even before Killer started. I bought a new litter box because Kiri would hide in the living room and use the corner because she was afraid to walk across the apartment. I started keeping them separated more and more, until eventually I kept Killer in my bedroom alone for a full week to see if she'd forget about the whole thing.

Kiri quickly became more confident with Killer locked up, but Killer would still hiss at the door if she knew Kiri was on the other side. Near the end of the week she seemed to be calming down so we slowly reintroduced them to each other and other than a couple of hisses they've been somewhat civil but not friendly.

However, soon after the reintroduction, Killer started vomiting up her food and kept doing it about once every couple of hours until it was just foam. It's become much less frequent, but I also don't think she's eating anywhere near as much as usual. She seems much less aggressive and more fearful, and now Kiri seems to be the one to hiss first when Killer is minding her own business.

I'm not really sure what to do; it seems pretty clear to me that the vomiting and lack of appetite on Killer's part is due to stress, and I don't want to stress her out more by taking her to a vet if there's nothing a vet could do. Ive tried those herbal calming drops and they didn't do anything. Should I keep them separated again? Do those calming collar work at all? What can I try feeding her that will settle her stomach?

Thanks so much in advance for your help and concern.

***EDIT***
I grew concerned after Killer refused to eat some of her favorite foods (raw chicken, nupro cat supplements, parmesan cheese, even junky cat treats!) so I tried syringe feeding her some water and mushed up kibble and she immediately vomited it back up. The vomit had a definite green tinge to it too. Definitely taking her to the vet tomorrow, if only to check on her hydration, but does this sound like anything in particular to anyone? An intestinal blockage maybe?