pain under left shoulder blade

I am googling this problem that began for me a week ago and I am seeing NOTHING to help answer my questions. Just lots of people posting about similar pain. Please what is the cause of it? Specifically, it is like a burning on the left top side of my back and my heating pad does not even reach all the way back there to totally relieve it. It comes and goes. I was good all day today, even while I was kick boxing. But by evening it was back with a vengeance. I feel good only when I am sitting back comfortably in a chair or sleeping. I try to keep good posture and I exercise. I have had a bad arthritic neck since I was late 30s (now I am a healthy 53 year old). I’m female. I have large breasts and am about size 13, a little overweight. I don’t feel this problem has anything, however, to do with my breasts being large,and they don’t seem affected by all this like many other reports I am reading on here, but I suppose large breasts could be a factor? Also I have degenerative disc disease but have been getting by fairly comfortably with it for the past eight years. HELP!! What can this be? (no good health insurance do I have). Andrea