I know very little about lung cancer, but I do know that pain along the shoulder blades (especially in between) often comes from degeneration of the spinal discs in your neck (weird, I know, since the pain is not in the neck.)
That was my first symptom. I was a smoker for 10 years (am 29 and quit 4 mos. ago) and found out that one of my spinal joints is arthritic. Since that time, I found out smoking is a risk factor for developing the condition earlier in life (usually it's a condition caused by aging).
Might be worth asking your doctor about, as he could give you x-rays or maybe even order an mri to see if that might be the cause.