I have just begun having this issue. I notice mine more at night. I have had normal blood tests, heart tests, all the things (tested for other reasons but all came back normal). My sleep study revealed severe sleep apnea but this issue is new so I’m not really correlating the two. I have a deviated septum but since using my cpap I actually have started breathing through my nose more on purpose. I wear a full mask cpap. I’ve been referring to my SOB as like breathing anxiety bc I feel anxious when it gets close to bedtime. In the past I’ve had recurring sinus infections which led me to constantly hocking up phlegm and it became a constant thing consistently recently last year which feels like there’s constant inflammation in my throat. I contribute that to Covid (thick elastic clinging phlegm).
I’m going to try those exercises you wrote about but I’m shocked at how many people have this issue.