What Causes Chronic Coughing and Mucous in the Throat?

For about the past 3 months, I have had a chronic cough and mucus or sputum coming up from my throat or chest. My throat always feels like it needs to be cleared, so i have to try to clear it or force myself to cough or hawk it up. I do not smoke. I did have a diagnosis of a very mild case of pnuemonia about 2 months ago. It always feels like there is blockage in my throat or and air bubble back there. My spit is always really thick. The mucus that comes up is a light yellowish or sometimes tannish, sometimes a bigger more yellow chunk will come up. I am constantly coughing or going to the sink to try to "hawk" up the blockage. Sometimes taking a deep breath makes me feel the need to cough. This makes it feel difficult to breathe properly, and it seems to hardly ever subside. I have also been tested for asthma and they said I did not have it. What could this be from?