My cat of 6 months is sick

My cat Ibiza is sick. Up to date on all her shots. 2 weeks ago we got her spaded which was the last thing. A week after I notice throw up, yellowish with food and didn't think anything of it. The day after that, I came home after work and there was way more throw up spots. Some yellow one and some white and foamy looking ones. I took her to the vet ER. They checked her vitals and she was running a fever 104.3. They had me fill out a questionnaire. I notice something about lilies on it and immediately text my gf asking what kind of flower we had on our kitchen table and of course they were lilies. I told the vet about this so they did a blood test and her kidneys were fine. I didn't see any sigh of lilies in her throw up so maybe just being next to them she got sick? They sent her home and gave me antibiotics. It is now day 5. She is still lethargic not eating or drinking on her own. I have been force feeding her. She is not throwing up. Moving around a little more day after day. Does anyone know or has had their cat with similar symptoms? Also, how long are cats usually sick for? We are taking her back to the vet tomorrow, wanted to see how she would be after all of the medicine was gone.

Most likely, she got sick from the stress of being at the vet's for the spay.  Since it's been 5 days since you took her to the ER vet and she's not significantly better, I'd get to your regular vet as soon as possible for a re-check and maybe a different antibiotic.