The "BUBBLE" you are feeling is probably the expander either buckled and went throuh the muscle fibers. or just the expander seperated the fibers of the muscle and now you feel it because of thin skin. With excercise you probably seperated the muscle fibers more.
Now repair depend on the examination, if there is buckling of expander and is not flattening with expansion, you may need surgery, becuse the buckled expander edge can erode through the skin.
If it is only seperation of the fibers because the general surgeon who did the mastectomy took out the fascia of the muscle, making it easy for the fibers to seperate, then continue with expansion slowly and at the time of exchange of the expander to the implantthat seperation can be re-enforced with Alloderm.
If the muscle is that bad and the local skin muscle of the chest is that bad then convert to a latismus dorsi flap or tRAM flap reconstruction.Your plastic surgeon should be able to examine you and discuss your options, and hi/her findings.