I too seem to have developed this condition in my left shoulder. I suddenly noticed it in the night about a week ago and it's got progressively worse during the last few days. It's quite painful and unrelenting. It's particularly bad at night and I haven't been able to get any sleep for three nights running, hence this nocturnal posting. I went to the doctor yesterday. I've been put on anti-inflammatories and told to see a physio if it doesn't improve over the next couple of days.
I don't know if this is connected with concertina playing. Firstly I don't get much time to practice anyway and I haven't been playing very much just lately. I certainly never noticed any discomfort when playing. On the other hand I can't remember suffering a particular trauma and haven't indulged in any other new activities that I can think of.
Pain and sleep deprivation drove me to the physio earlier than planned. The good news is that this condition was probably caused by my sleeping position (on front with arms tucked under head), which is not advised for older people such as myself! My arm bone acted like some sort of lever crushing the bursa, a sack that lies between the arm bone and the underside of the shoulder. This is known as bursitis. A doctor could explain this more accurately but you get the general idea. The bad news is that it will probably take quite a long time before I can get back to playing.
So although I thought I might be 'suffering for my art' it appears that it is now safe to go back into the water...