I'm not quite sure from your question whether the thickening on your chest wall is NEW, or what type of changes in the tissue under your arm you are experiencing...
If these are new symptoms, I think it would be a good idea to schedule an evaluation with a breast specialist. (In my opinion, it's always better to err on the side of caution.)
Then, if you are cleared regarding breast cancer concerns, physical therapy could well be helpful.
Beest wishes...
I'm not sure unless it's a matter of aging and / or inactivity. You might consider seeing a Physical Therapist for an evaluation and possible treatment program. If after evaluation if any testing is necessary I'm sure the Therapist would advise you appropriately. Regards ....