My mom had shoulder pain for at least a year and the doctors kept telling her it was a muscle strain, or soft tissue injury. Finally, an x-ray was done of the shoulder and they found a huge tumor in her lung. After further scans and such, she was dx with stage 4 non-small cell lung cancer with met to bone and brain. She had NO SYMPTOMS except shoulder pain. So the possibility is there. I would really check it out. Good luck!
Gretchen (Motherhen)