My cat won't stop sneezing!

My cat just left the vet two days ago after getting treated for a urinary blockage. He is on antibiotics and a special compound to open his urethra. Since he has been home, he has been sneezing, and it concerns me because I've NEVER heard him sneeze. Do you think he caught something at the vet, or could he have allergies? My husband looked up the antibiotics and it said they cause "flu like symptoms", maybe that's it? Any response is appreciated!

It could just be the medication, but there's always a risk of URI when there are several cats in a confined area which may or may not be sick already. Wait it out unless it gets worse. 

One of my cats does this sometimes with no other symptoms. If you are concerned about this being the start of a URI then you could try the steam treatment. Run the shower as hot as possible to steam up the bathroom. Sit with your cat in the bathroom for 15-20 minutes 2-3 times a day. This may help break up congestion if he is starting to get something.