Apparently this is what was causing me to feel weird when taking a deep breath. The doctor I saw last week correctly noted that it was not a broken rib — which was my first suspicion — but he said he thought it was a muscular issue, especially with the muscles connecting the pelvis to the ribcage. Last night, I finally had had enough and went to the hospital, and the patient, friendly doctor checked me carefully (even offering to do an electrocardiogram, though I’ve had a few of those in the last few months and they’ve always been okay) and came to the conclusion that it was probably costochondritis that was bothering me.
Costochondritis is basically just an inflammation of the cartilage in the ribcage, and something not uncommon after a cold or other infection. That would also explain the (much milder) discomfort I was having on my left side, since that was also in the area of the ribcage that is cartilagenous, rather than bone.
It was when a nurse gave me a shot that I was struck by the wonder that is modern medicine. A single shot, and in a few minutes my breathing was all but back to normal. The oral medication I have now is less effective, or else I’m feeling secondary effects — tension in my abdominal muscles, probably from tensing up for the last week or so, out of a weird feeling that my guts would burst if I breathed too deeply, and a heightened sensitivity to what’s going on inside my guts — but in general I feel okay again.
There were other indicators, though, that reminded me that I need to get my health under control. Happily, I should be able to get on with that soon.