Am on z pack for bronchitis. when i turn over to left side in bed, my cough gets much worse. why?

Pressure on the lung: It is possible that you have more than bronchitis, such pneumonia on the left side. When laying on bed, the bed is exerting pressure on the chest wall and thus pressure on the lung tissue as well. If the areas is inflammed, it is senstive and thus may worsen your cough. Since you are being treated, just take it easy for few days--lay on back or inclined in couch may help. Consult doc. Good luck.