I am sorry to hear that Katrina seems to be having difficulty walking.
I understand that you are concerned that it may be related to the antibiotic that she is taking.
The short answer is that her difficulty walking is unlikely related to the antibiotic that she is taking. There are some antibiotics (for example Gentamicin) that can affect the balance system, but they aren't likely to be the ones that she would take for a respiratory infection.
What antibiotic is she taking?
My thought is that her upper respiratory infection has led to a plugged inner ear or a secondary bacterial infection in her inner ear and that is affecting her balance, thus she is having trouble walking.
Is her head tilted to the right or left?
When she tries to walk is she circling or leaning to one side or walking very slowly with a wide stance and then leaning and falling over?
Does she have rhythmic back and forth or circular eye movement?
(like this dog : http://www.youtube.com/watch?v=NaB7OJRLVQ4 ).
If the answer to these questions is yes then she may have vestibular disease. Vestibular disease is a malfunction of the balance system, either a problem in the inner ear itself or in the nerves that take information to the brain or in the brain itself. Episodes often come on suddenly.
With vestibular disease she can get very dizzy and as such have trouble with coordinated movement. They will often fall or lean to one side.
There can be several causes of vestibular disease. They range from very benign causes such as idiopathic (meaning we don't know the cause but they resolve on their own with supportive care) to middle ear inflammation, infections or polyps, brain infections (bacterial, fungal or viral) or even a primary brain lesion such as a blood clot, bleeding or a tumor.
If we cannot identify a cause then we will often treat the patient symptomatically (anti-nausea drugs, anti-inflammatories and possibly antibiotics if an ear infection is a concern) as most dogs do get better with supportive care.
Her prognosis if this is caused by a lesion outside the brain (such as an upper respiratory infection) is very good in most cases.
We may check bloodwork to make sure organ failure or thyroid hormone abnormalities are not the cause of her symptoms. If she isn't improving in 3 to 5 days then blood tests should be done to make sure all is well internally.
At home you can give Gravol also known as Dramamine (dimenhydranate) to control nausea, which is also used for carsickness. The dose is 4mg to 8mg per pound of body weight every 8 hours. Side effects are mild sleepiness and dry mouth.
Or you can try Benadryl (diphenhydramine only, do not use products with acetaminophen or decongestants as they are toxic for cats) at a dose of 1mg to 2mg per pound of body weight orally every 12 hours. That's 1/2 of a 25mg tablet per 8-15 pounds of body weight every 12 hours. Side effects are sedation and dry mouth as well. Benadryl is very bitter and some cats will drool excessively or may even vomit because of that. That doesn't mean it is making her ill, she just hates the taste. If that's the case with her try Dramamine instead.
The other possibility, if she doesn't seem to have a head tilt or be off balance but rather just seems painful, is that her upper respiratory infection may caused by Calici virus. That virus can cause joint cartilage erosions, swelling, and pain, making it painful for them to walk and move around. If that is the case perhaps a prescription for pain medication, and keeping her quiet and confined will be the answer.
Please let me know if you have any further questions.