Hope it feels better soon, Janet. A few months ago, I, too, had a mystifying bout of pain in my left shoulder - the "good" shoulder as it's the right one that was surgically reconstructed after a fracture. I kept moving it in circles and stretching and eventually it was fine. I did get a new backpack (for commuting) and shifted some weight from my across-the-body shoulder bag somewhere during that time period, but there wasn't an immediate effect from that, and I'd been carrying that stuff around for years, so I'm not sure it had anything to do with it at all. There just wasn't a clear cause which was frustrating.
I'm sure my cross body bag doesn't help so I got a couple new ones to try along with a smaller lighter wallet plus card case. We'll see.