The gunk thing could be it, im not really ruling anything out at this point, some things just seem less likely then others. The gunk thing mainly because its day one or two of using the new build, just doesnt seem it would be gunked up that fast, but then again with the excess juice issue.
I quit smoking a long time ago, and had some issues at first, but they cleared up a week or two into vaping. The settings and stuff i am not changing, usually just toss it on the 7watt ipv and vape like usual, just occasionally my chest doesnt like it.
You are pretty close with the running thing, that is sort of how it feels, if my out of shape ... runs and my chest starts hurting/burning, i would compare it more towards a chest cold, congestion, bronchitis type burn though, and it clears up fast once i change the build, the thing thats getting me is if all i am doing is rebuilding, not changing juices or settings, then why does it go away?
That's how mild lung irritants start, they come and go, I battled with it till I started DIY juice and lowered my flavoring %, your lungs aren't telling you to change your coil / wick, you could even possibly be forming an allergy to pg or vg and not even realize it ..................