Correct. Giving antibiotics without knowing what you're dealing with isn't usually a good idea. The next time you get feed, make sure you take note of what brand and protein level. Most grower feeds have 20% which is what you want. You might want to pick up some Rooster Booster vitamins & probiotics. You just put 1/3 teaspoon in a gallon of water and mix fresh daily. I wouldn't add it for more than 4 or 5 days though, as it also has electrolytes and you don't need to overdo that. Another option is to pick up some Poly Vi Sol baby vitamins without iron and give her 2 or 3 drops daily for a few days. It's a very well rounded vitamin supplement. My girls get it once a week mixed in their Kaytee baby bird food that I mix with water to pudding consistency. They go nuts for it.
How is her breathing? Is there any drainage from her eyes or nostrils? Do her poops look normal? Can you define the symptoms that make you think she has the sniffles? Poor gal. Hoping she perks up.