Most likely a post-operative sinus infection, which is very common after
sinus surgery.

It is really impossible to create a sterile field during and immediately
after sinus surgery.  It's inherently "dirty" because the dried blood
from the surgery makes a growth medium for bacteria, and the nasal mucus
is trapping all the dirt and germs you breathe in from the air.  (Like
the filter on your air conditioner.)

Also, the sinus surgery has damaged your cilia, which are the mechanism
 to sweep out germs and dirt from the sinuses and nose.  That makes you
more susceptible to infection until the cilia regenerate.