I can relate to this. Seems I was infected with mycoplasmas when I was 11 years old. I had some pain in my shoulders, but since I was mostly around older people, just thought it was normal. By the 9th grade my shoulders would fall out of joint. I never really developed pectoral muscles either. I don't know if the things I was infected with had cut off blood and/or nerve flow to that area or just what the mechanics of it is.
With Brachial Neuritis, there is normally a short period of pain followed by weakness. I have been diagnosed with this. I had some pain for about 3 weeks and only after that did weakness set in. I was weak from March until August without pain and finally went to a neurologist who diagnosed me with BN. It is very rare, but I also had another auto-immune disease, Guillain Barre Syndrome in 2007 so it may not be so odd that my immune system mis-behaved again.