Cat, yeast infection in ears

[superkat]

Hello everyone, I am new here and I am desperately looking for answers.
My cat had a dental treatment 6 days ago (several teeth were taken out) and he was prescribed antibiotic drops twice a day for 10 days (clindamycin hydrochloride 1cc every 12h) and transdermal gel with codeine 0.1cc twice a day spread inside inner part of the ear (switch ears). When I took the cat from the vet, his ears were more pink then usual, but I didn't pay much attention. After 4 days I noticed brown wax in both ears (we never had any ear infection before), my first thought was like maybe some dust set on the gel, I cleaned one ear, next day the brown wax came back. I googled and found the picture of cat with yeast ears infection - exactly same picture that I see on my cat's ears.
Did anyone had the same bad experience, can it be connected to medicines that I give him or could the cat be infected in the hospital?
I am going to take him to the hospital tonight but I would like to get some answers before I go there. Thank you.