They did show something. They showed you don't have the flu, strep throat, or a chest infection (which is a sign of pneumonia). So he learned a lot!
You have an infection because of all your symptoms. (You don't get those symptoms unless you have an infection.) They are caused either by bacteria or virus. He can't tell if it bacterial or viral with the tests he did, but he gave you Zithromax just in case it is bacterial. It is bacterial, the medicine will speed your recovery. If it is viral, the medicine will not help and you will just get over it at your own pace. The best you can do is do whatever you can to boost your immune system, which will help with both viral and bacterial. For example, get lots of rest, eat well (if you even feel like eating).

Hope you feel better. I'm sick too.