Good news about the stress test and lung x-ray. What you are experiencing could be caused by either sleep apnea and/or nocturnal asthma. You could also be having gastroesophageal reflux (GERD) and that could certainly aggravate any pre-existing asthma. Incidentally untreated sleep apnea can, itself, aggravate nocturnal asthma.
One way to approach this is to see your doctor and obtain a peak-flow meter, a small hand held device that can be used to assess asthma. Then start recording peak-flow values 4 times a day: immediately on awakening in the morning, at noon, at 6:00 PM and at bedtime. If this is nocturnal asthma you may see a significant fall in peak flow overnight; that is, a drop from the flow rate at bedtime to that in the early morning.
So, you should ask your doctor if he/she believes this is asthma, if you have GERD and if, perhaps it might be time to reevaluate your sleep apnea.
I wanted to add that I was NOT diagnosed with asthma, just wondered if what I was experiencing could be asthma. Just wanted to clarify.