What Causes Stabbing Pain On The Shoulder While Suffering From Multiple Sclerosis?

Good morning. Since January, at random, usually beginning 8:45 to 11:00 p.m., I have episodes of severe, knifing pain in my right shoulder/armpit that last around 45 minutes. I have multiple sclerosis (21 years) and currently take the oral medication Gilenya. I have seen neurology and orthopedic doctors, had physical therapy (which seemed to temporarily help), and MRI, X-ray and blood work done, all which were negative. No on knows what this is or what causes it. It started after a motorcycle ride that I got extremely cold on; I'm wondering if it could be nerve/some other damage due to frostbite? What do you feel that it could be, and what do you think would help?