What Causes Coughing Up Of Green Coloured Phlegm In Mornings?

Hello Doctor, I have been living through some kind of infection since mid-February and I cannot figure out what is happening. I waited a month before I visited my doctor because I thought it would go away on its own. Every morning I wake up and I cough up green phlegm. The phlegm was really thick and chunky at one point a few weeks ago, but now it is more thin and lighter in color. Throughout the day, I do bring up more phlegm, but it varies between light green to clear in color. I also have a random cough that comes and goes - I can go for hours without a cough, and then have a whole bunch of coughs within a 5 minute period, only to be fine for the next several hours. Right now, I am experiencing some chest tightness, but that only started when I had a coughing fit on Friday (that was the only coughing fit I have had since catching whatever this is). When I first saw the doctor (not my regular doctor) he said I had a throat and sinus infection and prescribed me amoxicillin. That did not work. Then I saw my regular family physician and he gave me Sdz-Azithromycin 250MG ( took 2 on the first day and 1 each day for 4 days after). That did not work either. Both doctors did listen to my lungs and said that my lungs were functioning normally and they did not hear any wheezing. I feel normal otherwise, I can still carry on as if it was not a problem. Today, I still have green phlegm. I am going to see him tomorrow to go through a bunch of tests, but what could this possibly be? Can you help me some light on what this problem may be? Automatically, my mind is assuming the worst!