Why isn't my cough going away?

I've had a chest cough for months, occasionally coughing up pleghm. suspected it was bronchitis as I'd been having fevers too. A couple of months ago I had erythromycin. And 5 days ago I started clarithromycin.. still not gone or minimized. Do I need to stay in bed all day for it to work, or something? I have been keeping on-the-go. I'm only 20, I don't smoke, I can't imagine what else it could be. Is it just one of those long viruses that take like a year to go away?