What Causes Breathing Difficulty After Double Mastectomy?

Hello; My 77 year old mother had a double mastectomy at the beginning of June. This was the second occurrence of cancer in the left breast and she made the decision to have both removed. Prior to this, she was in very good health and at a very healthy weight. They harvested lymph nodes and we are now in the process of finding out how far (and to which areas) it has spread. My concern is that she has had an increasingly difficult time getting in air...she can t walk further than a few feet before she has to catch her breath. It has been getting worse over the last few weeks. She is tired all the time and naps at least twice a day in addition to sleeping 4-6 hours each night. Does anyone know if we should be worried? Thank you,