I'm sorry I haven't any experience of the same thing. My dog has a lump IN the muscle of her shoulder. She has had this for many years. I took her for it to be examined, and it is a Lipoma (a benign fatty tumor) It is harmless, doesn't need to be removed and it doesn't affect her movement. But I don't think this is the same thing as you described.
You will have to wait on those biopsy results to know exactly what this is, and what treatment is needed.