[CooperBoy]

Hi, my 4-month old Rottie (male, named Cooper) recently had a cold with a snotty nose (green and yellow snot), sneezing, etc. We took him to the vet and got an antibiotic. The vet was treating our dog like he's a big dog (older than he really was - 3.5 months at that time) because Cooper is bigger than a regular puppy - he's 30 pounds now (regular puppy, meaning other breed puppy). So, I decided to give Cooper 1/2 the doze of the antibiotic - the vet gave me enough for 10 days, supposedly 1 table in the morning and 1 table at night. I was giving Cooper 1/2 tablet instead, since it was his 1st antibiotic (I made sure he had enough plain yogurt too, to coat his stomach - we would put the yogurt on his food). 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 vet decided to resume Cooper's vaccine routine - Cooper had the rabies and the kennel cough vaccine in one day. It seems like ever since then his cold is back. The vaccine took place a week ago, and Cooper is all stuffed up, sneezing and doing that "hyperventilating"-looking thing few times a day. He doesn't have the snot coming out like he did before, but he seems congested.
What do I do? I still have some of that antibiotic left - 4 days worth of full dose, so I started giving him that because it seems exactly like that cold he had.
Anyway, has anyone heard of their/or any other dog that does that hyperventilating?
Thanks