What Could Have Caused This Nasty Cough?

Question

I have been suffering from cough for a long time and tried to control it with various over the counter medications. This was quite useless really and finally the cough worsened into bronchitis and I had to go see a doctor. I was given some medication which I will be completing soon. After that the doctor wants to track down the triggers responsible for my persistent cough. He wants me to maintain a log and enter when, why and what causes my cough. This I have to do for the next two weeks. He had ordered a chest X-ray to be taken which showed a clear chest. After 2 weeks he wants me to go for a pulmonary test. He says I could be allergic to something. I’ve never smoked so quite perplexed as to what’s causing this nasty cough.  I’m still searching through the net for various reasons. Any experienced people out there with any idea about what I’m going through?