What Cause Sore Throat And Congestion?

I have been sick for over 3 weeks with a soar throat and congestion. Now I am coughing and it feels as if it is in my chest and I am having bad back pain. It is not a muscle strained back pain but worse. I went to the doctor yester day and he gave me a z-pack and a steroid shot but did not inform me of what I have. Can you please help me figure out what is wrong with me where I can get better?