I wonder if you can duplicate it by moving your arms or stretching that area in one way or another? I'd say it's probably a muscle strain, more than a lung thing. If you can duplicate it by moving or stretching, then it's probably a muscle strain.

I get strains in my ribs for strange or unknown reasons, usually after physical activity, but sometimes after sleep. I can feel them when I breathe because my rib cage is moving. Costochondritis, is what it's called, strain of the spaces between the ribs.

Karen