26 y/o female with constant pain/tightness between shoulder blades & neck. Heat/ice/massage helps only temporarily. What could be the cause?

Posture. while,there are other causes, posture with head forward position is a common cause in young women, especially if,she has large breasts, and rounds shoulders to hide it. See your PMR or ortho doc to evaluate posture, image your neck, and help you get exercises or other treatments to help resolve the problem.