The danger also lies in the fact that by the time your kitty comes down with another URI, the medication prescribed previously may have expired ... or is due to a bacterial strain that the medication for the previous infection won't address. It is always safer to have the vet assess and recommend treatment when your cats are sick. Home-diagnosis and treatment is not a good idea.
I'm not a vet but from personal experience I would get her to the vet right away. My cat Princess has almost died several times due to URIs. I even had her at the vet the same day she showed any sign of an infection. She has a couple other issues now that make it worse when she gets sick so I have to be extra careful. I'm not trying to scare you. I'm just telling you my experiences. I hope your kitty feels better soon.