Just read through this thread, the pain in my left arm has today been diagnosed as a combination of tennis and golfers elbow (not that I play either, on affects the inside of the sow, the other the outside. Apparently!). Given some exercises to do, bought a tennis elbow clasp to wear when gigging and more tubes of ibugel (anti inflammatory ointment, not an apple trumpet).
Physio suggested narrower necked basses might help, great news when I've recently bought a Classic 50s Precison with a 1 3/4" nut! Might have to blow the dust off a jazz bass for this week's gigs.
Strongly suspect that it's because I've been doing all my heavy lifting (cabs, etc) with my left arm due to knackering my right wrist in a bike accident 2 years ago.