When to go back to the vet?

I took my cat (10 yr old neutered male) to the vet this morning after he threw up a ton had a major behavioral shift. I took him almost immediately and didn’t give a lot of time for symptoms to develop or anything like that. I’m feeling like I jumped the gun on the vet and wasn’t able to give them the full picture because of that.

They did a test on his bladder and kidneys an everything came up clear. Did a general exam and didn’t find anything. Said he had a minor fever they gave him an anti nausea shot and some b12. They gave me a weeks worth of antibiotics and said to monitor him over the next few days. They basically said they think he has a minor sickness of some sort and that if the antibiotics don’t help I can bring him back for a follow up in the following days.

I just gave him his first dose of antibiotics when I got home from work. But it seems like his condition is a little worse.

I am a worrier I worry about him often especially as he gets older. I don’t know at what point it’s just me worrying and at what point I need to take him to the vet to get another look at him.

The vet is accessible to me they have appointments frequently and also have walk up hours for situations like this. I live in the middle of nowhere Texas. Vet care is inexpensive here compared to major city’s and I can afford to get him help probably up to the $2,000 mark if it came to that.

TDLR: I think my cat’s condition may have worsened since I saw the vet this morning, but he just got his first dose of antibiotics. How soon is too soon to take him back.