Many persons new to the concertina or accordion comment on pains and soreness. Some say that it was muscle fatigue and went away before too long. Others are not so lucky. Accordion players commonly mention back problems from heavy instruments, for example. See a medical professional if you are having problems.
Every case is different.
The purpose here is to give one case study and show that how it was solved. Your problem, and solution, will likely vary. But you have company if playing a musical instrument is causing physical problems, and the resources listed below might help you. For a few weeks I didn't know why I was uncomfortable. I finally consulted my family doctor, who immediately suspected a problem with the carpal nerve.