I just stopped smoking. i feel like i have to dry hack. how come?

Several reasons: First, you might be sick. However, your lungs cleansing mechanisms are turning back on and you might be playing catch up.

Cilia Waking Up: When you smoke you deaden the lungs little lining of cilia which are like little waving fingers lining your lungs. Cilia work like vacuum cleaners to work mucous upwards out of your chest. When you quit smoking your cilia naturally start working again to clean the lungs which is why a cough gets activated. This is actually a good thing! Weakened cilia is one of the damaging effects of smoking!