I've had pain in my upper back and shoulder blades for over a year and it's getting much worse. what could be causing this?

Muscle spasm: A teenage female with upper back pain for over one year likely has chronic muscle spasm in this region. Underlying problems could be a mild scoliosis, large breasts and anterior sloped shoulders, an underlying pelvic girdle disorder, and abnormal postures and repetitive activities at work. A workplace ergonomic evaluation and the opinion of an osteopath are warranted.

Many Possibilities: Given that this has been going on for so long, i would consider an evaluation by a spine specialist. Sometimes it could be a muscle imbalance, early onset arthritis or scoliosis. Also could be a pinched nerve/herniated disc. Either way it might be good to figure it out and get appropriate treatment.