I hope you did not put her with the rest of your flock when you got home. If it were me, I'd take her right back to where you got her, it sounds as if she may have MG. This will infect any birds exposed to her.
With MG, Coryza and some of the other nasties, there doesn't have to be any symptoms at all. Scary huh? Smell near her nostrils, any foaming in the eyes? Sweaty under her wings? Coryza most probably. Sneezing, coughing, rattle when she breathes, are some of the symptoms of MG. Tylan 50 is one that can be used and you could also try a Duramycin-10 in their water. But I really think you should take them back where you got them. You were sold sick birds and you are risking total wipe out of what you had already as well as these new ones. This choice is yours to make. You can treat and probably will not have total wipe out but if it's one of these diseases, your birds will always have it and you really should close your flock now. Nothing in and nothing out. You can't sell any of your birds, you can't sell any of the eggs for hatching. To do so would put someone elses flock at risk. If you ever wanted to bring in new birds you would need to get all of your birds vaccinated before exposure to your birds. This is done during week 1 up to week 10 and then again in 4 weeks. This is for the MG and it is a very pricey vaccine as well. I bought it at 91.45 from Jeffer's last year to discover I didn't need it.
Slow down and take a deep breath! It's possible you did but if it were me I would have them all on Duramycin-10 right now. I would go with the 400mg dosing and give it to them for the 7 - 14 days. I don't know if you can beat it or not but it's worth a try at least.
At my Tractor Supply it's around 6.00. It's not much.
