Cat hacking a lot?

At first we thought he swallowed his spit wrong because he was sleeping then he suddenly woke up making a hack sounds. He and our other cat just had a respetory infection and was givin meds then got spayed and neutered. So we just something over 300 in one week for vet vists and can t really afford to go again for another couple of weeks. Any clue what could be going on? He s still very active, we thought he was trying to cough a hair ball and was having trouble because he s so young. I just want a general idea of what s going on so when I get to take him I can ask the vet everything.