Tylan or the other water soluble antibiotics will clear up most upper respiratory pretty quick, but I recommend you do a bit of diagnostic investigation first. (remember tho, if a medication doesnt make a remarkable difference in 48 hours, it is the WRONG medicine)
Check the eyes. If there is any eye involvement water soluble's generally wont work.
Smell the chicken.... I know, this sounds wierd, but if you take a whiff around the chickens head, and smell something bad... well it isnt bad breath! That is infectious Coryza, and it is everywhere this year! I have so many people calling or emailing me with sneezy chickens! the only thing that will work on that, according to the books, and my vet, is sulfadimethoxine. Yes that spelling is correct. there is a variety of it for dogs, that is NOT the stuff you want. you are looking for the water soluble packet that is labeled for chickens.
Works miracles, and the sneezing and swollen eyes are gone in days.
Sadly. Once that bug is in your flock, unless you cull everyone, and sterilize and leave things empty for a while, you have got it for good. Nasty nasty infection. But it can be handled, and responsibly treated.
So go give your girl a sniff, and keep your fingers crossed that she just smells like a chicken....
Good Luck.