I am experiencing the same, its on the left shoulder blade to the underarm ,rib cage.
I am also worried about it, if I have cancer.
First things first, go to your GP to get it checked out. Earlier this year, I had an awful pain radiating from under my rib to my under arm on my right hand side. My GP and the hospital said it could have been 1 of 3 things; 1) a pulled muscle, 2) gall stones, 3) liver cancer. I say go to your GP to get it checked out as your diagnosis was not on my list. By the way, I couldn't walk with the pain and I needed morphine to get rid of it.
Best wishes