Hi ArtEren.
Look into Adhesive Capsulitis a.c.a. frozen shoulder.
You are in the 1st phase, which is the pain phase and it's followed by
the stiffness phase and then finally the resolution phase, if it is indeed that.
Just look it up and see if the information you find on it is consistent with your experience so far, as there are other possibilities.
Hope it helps, however my suggestions and comments are not intended as a replacement for medical advice.
Cheers.
Niko
It may not be, but maybe dehydration, or can be what I dealt with which was neck disks that herniated and once they wrapped around the spinal canal and were pinching nerves, my left arm felt like lightening pain and numbness down my left arm. I had c4-7 spinal fusions. If that pain continues for more then a few days, see your doctor. A person doesn't have to have a fall or accident to have a herniated disk.