8 month old kitten with URI - Questions?

No, they don't usually continue getting worse while on antibiotics, as it sounds like your cat is. 6 days or so she's been on antibiotics? It could be something else such as asthma, that is coincidentally showing up at the same time or is flaring up due to the URI. There are other things that can cause wheezing also that may be her adjusting to your home. Low levels of humidity or second hand smoke are two common things that could be considered. One easy thing to do until you get to the vet is use a humidifier where she spends most of her time.