What Causes Pain Below Shoulder Blade And Cramping Seizure On Arm?

Recently I have been getting a sharp pain just below my shoulder blade, sometimes on the right side and sometimes on the left. Whichever side it happens on my arm on that side goes weak. I do not have sensory nerves in either arm (birth defect) so can not say whether it also goes numb or not. This has been going on for about 6 months at random times and irregardless of what type of activity I am doing at the time. Most recently this symptom advanced to a searing cramping seizure on my right arm right after the sharp pain beneath the shoulder blade that stayed seized and cramped up for 20 minutes and was severe enough to pull a muscle in my forearm. My original suspicion was a pinched nerve of some variety but the cramp/seizure of the arm is not really conducive to my original thoughts. Any ideas?