I believe my puppy has a respiratory infection. he is on two different meds but now he cant breathe.?

Should I take him back to the vet? They thought he might have had puppy distemper but tried to rule it out with the infection. It's been about 2 weeks on meds and he is more playful but still lethargic. Now he is gasping for breath and while sleeping he shakes with every breath of air, like its hard to breathe. He cries all the time too. I just feel like the vet doesn't know what's wrong with him.