First you need to rule out infection. The hospital probably ruled that out if you are here.
Your chords can be tightening from the mucus as well. Doesn't sound to hard to treat and pretty common.
First you need to find out if you have developed any allergies. They can come out later in life. A cheap way of doing this is a blood test for something called igE. If present you have allergies. Asthma can do this too. But if you are sure it is not asthma then look at allergies.
Do you have sinus mucus? If so try an ENT doc.
Finally you can go to a pulmnologist if your problem is only in your lungs. This is possible too. Some people have conditions of fluid in their lungs. Another reason not to pick up smoking.
I think you will find answers. I take antihistamine to help my mucus but mine is from allergies. Don't know about you. You will need to go to a few docs to lean about it.
I think some of it will be trial and error until they identify it.