<<Disclaimer: Verify this information before applying it to your situation.>>
 
Not medical advice
after a mastectomy a rare complication is lymphedema of the arm on the
same side.  this a blockage of the lymph channels in the arm pit.  It may
be helpful to maintain great care in protecting that arm from injuries,
cuts etc. Elevation might help. aproblem like should be discussed with
the doctor/ surgeon who did the operation.
Joe Murray