I pulled up your thread ....
http://www.rabbitsonline.net/view_topic.php?id=52681&forum_id=16
All the answers to your question are in this thread.
if the injection was indeed baytril ( and even if it was a one time dose of penicillin) there is a good chance of a flare-up. I dealt with this several years ago with a bun with a recurring URI. the vets here would not not use anything but baytril which was effective for the 1st few flare ups but then became virtually useless with the bun going into full blown symptoms while on it.
the injectable baytril is preferred over the oral however needs to be buffered prior to injection due to it's iirritating effect on tissue. it is really not the drug of choice which was pointed out in your previous thread.
Randy mentioned several other more effective drugs including zithromax ( azithromycin,) chloramphenical which are given orally
.In another recent thread he mention Zeniquin and Convenia
Convenia is given about once week for a rabbit per injection and is off label for rabbits but effective
First of all your vet needs to do a culturetodetermine the antibiotics that will be the most specific against the bacteiapresent in a culture
my bun responded most positively to bicillin injections which is a combo of benzathine and procaine penciallin which is given every other day subqutaneously.
A few sneezes does not mean a relapse but certainly something that you should monitor
you may need a vet who is better educated in rabbit medicine. (this can be hard to find)