The major reason that "we" feel this pain is because in "our" sleep we tend to cough less unless we are in the state of an exhasterbation...the coughing in the day is more severe when we wake up or sit up after a long period laying down in a flat way(on a bed, couch, the floor) because the sputum has settled in a new way, we also experience this when we go to bed and go from standing up to laying down because again the sputum has gone from being up right to parallel with the bed. Example take a gallon of milk, with the top on, and turn it on it's side with the handle down, the milk fills in that handle and will drain when set back upright...if you were to leave this for long enough for the milk to become partially curtled you would see milk residue "stuck" to the sides and in that handle. This is a great way to think of the sputum in your lungs, the sputum is able to fill in new areas of the lungs when you lay down and it causes you to cough in an attempt to keep the sputum out of those areas. It is a failed attempt because the sputum is too thick to be coughed out, or at least most of it is, The same thing happens when you stand up but it is now draining out of these areas and the movement is blocking areas that were able to allow oxygen that cannot now. I hope that helpes a little...the pain is actually where your muscles have started to repair the damage done by the strain placed on them by all the coughing you have done the day before, as you get sick the coughing becomes more intense and the "pain" level goes up. When you fend off the infection the pain will lesson back down to the "normal" pain level.
