15 years is a long time you have been bearing the pain. It would be definitely better to get one MRI of shoulder joint to identify the root cause of pain. How frequently did the episodes of pain occur. Currently, since how many days has the pain increased.
Next Steps
Start hot fomentation, take analgesic with muscle relaxant tablet, apply analgesic ointment locally.
Health Tips
Avoid heavy work, avoid jerky movements of neck.