Cough with lying down, medically termed orthopnea, is often associated with some degree of congestive heart failure. the accompanying symptoms of lower leg/ ankle swelling, a sense of abdominal fullness or early satiety as well as waking in the middle of the night very short of breath (a condition called paroxysmal nocturnal dyspnea) are further evidence that heart failure may be the cause of the cough. However, this is not a diagnosis that can be made based on one symptom and depending on your age and other risk factors (obesity, family history, diabetes, coronary artery disease), this diagnosis may be more or less likely. Another viable explanation for your symptoms is that this cough is attributable to your asthma
, although a wet cough is not as common as a dry cough and wheezing for asthma flares. Finally, depending on the course of the symptoms (ie did the cough come on all of a sudden, perhaps in the context of other cold symptoms they may have now resolved) this could just be a lingering post infectious cough that is worse at night. if the cough persists or worsens, you should go see your doctor
for a more thorough evaluation. hope this helps!