One question why do you have to add to pushing up the volume and by default the speed by playing your banjo - stick to the mando if that is what you are most comfortable with, do you really need to be heard above the rest? I don't mean to sound critical but if this is contributing to your physical discomfort and your alternative is to quit playing then change.

Also I suffered some similar pain a while ago, it was caused by tension and a rib being slightly misplaced a couple of trips to the Osteopath soon cured it - it cost a few quid but was well worth it.

Good luck