i had a cold for the best part of two months in Nov/December. turned out I'd had a truly horrible cold and then developed a chest infection (with same said catarrh). when I went to see the doctor about it, he asked me whether I'd had a bit of a lull in the middle when I thought I'd recovered before the cough came back (I had), and he said that was a classic sign of a chest infection - you get rid of the cold virus, but there's a bacterial infection which your body needs a bit of hand to deal with. He put me on anti-biotics and i was better within a week.
Go to the docs!