I had this four months ago. I haven't had an x ray yet. The first doctor said it was bronchitis. She gave me flixotide but it didn't work. It was so bad I felt as if I was going to die. the linctus didn't help and after going to a private doctor he diagnosed me with asthma (based on the fact i was previously diagnosed with exercise-induced asthma and some wheezing) and nebulized me.
The cough has never returned.
The tightness in my chest has returned though and sore, painful throat. It is because of this I will return to the doctor in an attempt to see if I have pneumonia, bronchitis or something horrible.
Do you also have acid reflux by any chance? Amazingly sometimes people with a chronic dry cough have acid reflux which triggers the cough. If everything else turns up negative, it might be worth a look. You can have it and not really have terrible symptoms of heartburn I guess. That is one question I get asked sometimes regarding a cough. Just a thought - best wishes for getting to the bottom of your symptoms.