Hi Gabriel,
How are you doing?
Are there any updates yet on your condition?
I read your post and thought I register to share some thoughts with everyone.
This condition started with me few months back, probably before announcing Coronavirus as a global pandemic.
3 things happened with me that I suggest were the reason this thing started with me:
1. I was having cold, so my nose was close, and I was exhaling strongly to get my nose open, so this might have done something inside my lungs.
2. I was a smoker, and quite suddenly, whereas should have quitted gradually, so that might be a reason.
3. I had a strong massage, after which things started, so it might be some pressure points or when the guy were pushing my back (facing lungs) hard, otherwise I was almost 1 hour lying on my abdomen for the back massage, so I suspect when he was massaging me, and turning me left and right, up and down, probably some acid from my stomach went backwards to lungs side.

Those 3 points are the main reasons I suspect one of them or two is the reason this phenomenon started with me, because between each one probably a week or two, I remember exactly, so I suspect them.

I was diagnosed with acid reflux, so I am currently taking medicine called Nexium 40, If any improvement happens, I will let you guys know.
The reason doctor said Acid Reflux is this shortness of breath + too much saliva + other symptoms.

I have done all heart diagnosis, other diagnosis, exhaling in a machine, Helicobacter pylori test, all were ok no problem.

By the way, during that period I was eating very little to reduce weight, so I wonder if this has anything to do with that, maybe the abdomen shrinks and causes acid to reflux to something!!

Other people says it might be IBS related "irritable bowel syndrome"
God knows what causes it guys, will keep you posted.