What Causes Persistent Cough With Green Phlegm?

I had a reall y bad cough for 2 to 3 weeks not stop couldn't sleep all night cough kind of thing. It started around Christmas, it started getting better a week ago and now it seems like it's trying to come back, but this time I have a light greenish phlegm that is coming up and it's really bitter. Almost makes me want to puke.