Upper Respiratory Infection & Throwing Up

My kitty had been sneezing repeatedly with a discharge and had for the most part stopped eating, but when he did, he threw up.  I took him to the vet and they did blood test and tested kidney and liver function which were all fine.  They gave him a antibiotic shot that they said would work for 10 days and he almost immediately starting acting better.  This was on Friday.  He stopped sneezing as much, no discharge, stopped throwing up and started eating again.  All was great.  On Tuesday night, he started sneezing again, stopped eating as much and is now  throwing up again.  Not sure where to go from here.  Blood test are fine, they say the antibiotic should continue to work, but he seems back to how he was before the vet on Friday.  Anyone have any ideas or similar situations?

His sneezing might be allergies vs Upper Respiratory Infection, so antibiotics might not help.  That could account for the sneezing,and with sneezing sometimes comes congestion (also could be allergy related, and when cats are congested they cannot smell their food, so don't want to eat.  The only thing that doesn't explain is the vomiting, UNLESS it's a foamy saliva type throw up, which is sometimes produced from excess stomach acid, which could come from an empty stomach. 

If any of this seems like it might be the case, you could contact your Vet and see if you could give an allergy pill (or part of one), and also try warming up some stinky canned food to try to get it to smell for him.  Barring that, you could take him to the Vet again for an anti-nausea shot and a another checkup.  Sometimes they can have blockages or other things that can cause nausea that won't show up in bloodwork.