Hi Eddzzz,
I have similar story. I didn't have any injury but I overworked my shoulder then got a lump just under my SC joint then my shoulder became kind of loose. SC joint becomes more irritated with the course of the day. I visited chiropractic, osteopath. I started doing stabilising exercises to help my shoulder a little bit. It's now 3rd month since all has started and I can't see any improvement. I am swimmer too and can say for sure it does hurt a little bit more after swimming but it's nothing sharp or acute.

The only hope I found on the internet seems to be microperforation prolotherapy or Platelet Rich Plasma Injections or mix of both Have you tried maybe any of these?

I hope you are better off now. If you could give me update on your situation, I would appreciate. I am looking for solution myself.

Agnes