In my experiences I have found that when I was younger and was only buying middies and regs that I would often get a cough after periods of heavy smoking. I believe this is due to the fact that one must smoke a great deal of cannabis to achieve that high when the quality is low, and the quantity smoked also must increase as your tolerance does. However recently I've stopped smoking regs and have been strictly smoking quality buds in which a few hits create a sufficient high, rather than 3-5 bowls. Basically what I'm saying is try buying higher quality cannabis and see if smoking a lesser quantity diminishes and eliminates coughing. It worked for me and it might work for you.

Higher quality = less carcinogens in the throat and longs due to a smaller amount being smoked.
Low quality = a large amount smoked along with a great deal more harmful materials being inhaled, thus causing irritation in the throat.

Or maybe you have a cold