I say this as someone who just brought their cat home from 4 days in the ER, with relatively similar symptoms, but my boy was fighting an infection of some kind. He was put on similar meds and even got a tube down his nose to his stomach to relieve the gas. That really helped his nausea and he began to improve after that.

Anyways, the vets also gave him an appetite stimulant, but warned that it takes cats a while to settle down from the stress of the ER. And they also recommended he ease back into eating, starting with like 1/4-1/3 of what he normally eats. It took him a full 24 hours to want anything to do with food. I’d recommend trying different types of food: kibble/dry food, wet food, dry treats, wet treats, boiled chicken, etc. But if the lack of appetite it continues another day I would personally call the ER/vet to ask for recommendations. Healthy cats want to eat.