Interesting. Almost sounds like a cracked rib... Sometimes ribs will start out feeling OK but the discomfort builds as you go through weeks and months of overcompensation with your muscular movements and the way you carry yourself. Definitely makes it hurt to cough or purge. Was there at any point an incident where such a thing could have happened?
The other idea would be a pulled muscle, like you suggested, or possibly a progressively pulled muscle group that is being exacerbated daily by some kind of nonoptimal ergonomic situation. Here's a great example: my wife writes electronic music and has this big fancy workstation with all these keyboards, monitors, equipment racks etc. Now she was well were that there was something amiss but no matter what we tried, including several modifications to the workstation, we just couldn't get the screen low enough and the computer keyboard high enough, so she was really aggravating her body. But she dealt with it and ignored it as much as possible. Finally one day she is feeling a chest pain, calls the ER, we end up running in there immediately for ten grand worth of cardio tests. Turns out she's healthy as an ox (she was around 34 at the time) and just had some pulled muscles. So she abandoned the workstation, simplified her setup to almost nothing, sits comfortably, and no longer has any problems. She is also working out 3-5 days a week, which provides essential core strength and balance.
It's probably just something with your musculoskeletal system... Nothing to panic about but definitely something to address.
Best of luck! This is a phase that will pass, remember that always.
