[jlhnokc]

"Anyway, he started getting better right away, but once in a while (every few days) he would do this thing where he would look like he's hyperventilating - breathing in and out really deep through his nose. Since he wasn't snotty anymore and he would only do that once in a great while and it was nothing severe,"

"The meds were working at that dosage, he was fine before the vaccines..."

No he wasn't fine. Your posts told us that...he was still look to be hyperventilation every once in a while. That's not fine. When you brought him back to the vet, did you tell him you cut the dosage in 1/2? I'm also curious why you would do that? If you don't have faith in your vet to give the right amounts of antibiotic, you need to go to a different vet. I'm with MoonDog, take him back to the vet, tell him what you did and go from there.