i had a lump under my armpit which over time turned into three or four of them and got extremely painful so i went to doc, it was a lymph node infection and cleared up with anti biotics, hope this helps!
I think you should go see your doctor. I too had an armpit lump, but for me it was only a month or so and it was an in-grown hair. I treated it a couple of times a day with camphophenique, which soothed the pain and eventually reduced the swelling. But, for you with that sore breast and shaky hand I would definitely get into the Doctor. The sooner the better.
My little sister had that exact same thing. It was a cyst. It wasnt cancerous, and she still has it, (she chose not to have it operated on). You should get it checked out though, for sure.