Hello! I know this is a LONG shot since you posted in 2020, but please if you see this~ did you ever figure out what was happening or improve on your own? I hope you did, and would love any info... I'm 29/otherwise healthy and have been experiencing this same thing for 6 months!! Had mono last fall but they never tested for any other infections. I've had very similar tests done - heart, gut, lungs, abdomen, allergies all checked with no weird results or answers... But still experiencing the feeling every afternoon, like a tightness or phlegm trapped centrally, regardless of diet or activity level or location, and no cough unless I try. No physical pain elsewhere. I'm starting to feel crazy and my doctor seems to not believe me, she's admitted she's stumped.