The doctor says I don't have one of these.
What it is not: a worsening of my congestive heart failure; anything to do with the lungs (they're clear); gall bladder; liver; an ingested alien. (These are goodness things.)
What it may be: anybody's guess.
Next stop: an upper-GI-peering-down-the-old-gullet routine. Scheduled for next Tuesday.