cat mouth breathing

Question
Our nearly 2 y/o male cat started acting funny (hiding, lethargic, little eating/drinking,) 2 days ago.  We took him to the vet.  No fever.  The did a UA, and said the concentration was fine and there was a "little" protein.  Said they suspected a UTI.  Started clavamox.  The next AM (after 2 doses of meds) he was breathing funny--resting w/his mouth open.  Not breathing rapidly or distressed.  Took him back and they thought he had an allergy to the meds.  Started him on orabax yesterday.  Eating better.  Much improved in the interest he's showing in things.  Playing better.  Still, tho, he's resting w/his mouth open.  Fairly wide.  But just keeps it that way.  He's not gasping or panting.  Seems to be breathing normally.  He's still pretty subdued in acting, but the open mouth thing is weird and disturbing.  Is that likely anything we need to be concerned about?

Answer
Open mouthed breathing is a method of increased scent detection in cats.  If he was partially blocked of normal breathing by virus or infection he could do this as a way to detect better.  He also could do this in response to medication he found unusual in scent/taste.  Has the behavior continued?

Humblest apologies for the delay as family illness kept me away for a time.