I have a lump in the muscle of my right upper arm. It is slightly bruised but only hurts when I pick up something heavy. The arm doesn't hurt if I raise it or lower it... only when I pick something up. I recently had shoulder pain but after an injection the pain went away. The knot appeared after I tried reaching slightly behind me with my right arm. The pain was excruciating. Right after that, the knot appeared. What could it be?