Would cortisone injection in neck/shoulder cause chest pains & infection 7 month after mastectomy?

This is most likely leftover fatty tissue. Your doctor can try to reduce the size of this area with liposuction. 

Three months is quite a long time to be draining continuously. Has your doctor checked for hidden infections? Has the fluid been adequately evacuated or is there always fluid sloshing around? Until the tissue surfaces can adhere to one another, they will keep leaking and re-filling the seroma...

You are right! Your menstrural cycle should not have any effect on MRI results following a bilateral mastectomy. Just have the test whenever it is convenient for your schedule. 

Some surgeons feel the blood supply to the nipple is strengthened if some of the tissue under the nipple is removed a few days/weeks before the actual Mastectomy. This is somewhat controversial as even when a nipple delay is performed, some surgeons still see their patients' nipples have poor...

More information on why this has been recommended would be helpful- are you positive for a genetic mutation? We don't recommend that pregnant patients undergo elective mastectomy surgery. 

Most likely this is a fluid collection or perhaps a collection of congealed blood called a hematoma. Neither are dangerous, but can make the site uncomfortable. While your body will take care of the swelling eventually, no matter the cause, you may feel better faster if a liquid portion can be...